Linux Virtualization - Feb 2013 - virtio PCI on KVM without IO BARs

OK we talked about this a while ago, here's
a summary and some proposals:
At the moment, virtio PCI uses IO BARs for all accesses.

The reason for IO use is the cost of different VM exit types
of transactions and their emulation on KVM on x86
(it would be trivial to use memory BARs on non x86 platforms
 if they don't have PIO).
Example benchmark (cycles per transaction):
	(io access) outw 1737
	(memory access) movw 4341
for comparison:
	(hypercall access): vmcall 1566
	(pv memory access) movw_fast 1817 (*explanation what this is below)

This creates a problem if we want to make virtio devices
proper PCI express devices with native hotplug support.
This is because each hotpluggable PCI express device always has
a PCI express port (port per device),
where each port is represented by a PCI to PCI bridge.
In turn, a PCI to PCI bridge claims a 4Kbyte aligned
range of IO addresses. This means that we can have at
most 15 such devices, this is a nasty limitation.

Another problem with PIO is support for physical virtio devices,
and nested virt: KVM currently programs all PIO accesses
to cause vm exit, so using this device in a VM will be slow.

So we really want to stop using IO BARs completely if at all possible,
but looking at the table above, switching to memory BAR and movw for
notifications will not work well.

Possible solutions:
1. hypercall instead of PIO
	basically add a hypercall that gets an MMIO address/data
	and does an MMIO write for us.
	We'll want some capability in the device to let guest know
		this is what it should do.
	Pros: even faster than PIO
	Cons: this won't help nested or assigned devices (won't hurt
	      them either as it will be conditional on the capability above).
	Cons: need host kernel support, which then has to be maintained
	      forever, even if intel speeds up MMIO exits.

2. pv memory access
	There are two reasons that memory access is slower:
		- one is that it's handled as an EPT misconfiguration error
		so handled by cpu slow path
		- one is that we need to decode the x86 instruction in
		software, to calculate address/data for the access.

	We could agree that guests would use a specific instruction
	for virtio accesses, and fast-path it specifically.
	This is the pv memory access option above.
	Pros: helps assigned devices and nested virt
	Pros: easy to drop if hardware support is there
	Cons: a bit slower than IO
	Cons: need host kernel support

3. hypervisor assigned IO address
	qemu can reserve IO addresses and assign to virtio devices.
	2 bytes per device (for notification and ISR access) will be
	enough. So we can reserve 4K and this gets us 2000 devices.
        From KVM perspective, nothing changes.
	We'll want some capability in the device to let guest know
	this is what it should do, and pass the io address.
	One way to reserve the addresses is by using the bridge.
	Pros: no need for host kernel support
	Pros: regular PIO so fast
	Cons: does not help assigned devices, breaks nested virt

Simply counting pros/cons, option 3 seems best. It's also the
easiest to implement.

Comments?

-- 
MST

On 2013-02-28 16:24, Michael S. Tsirkin wrote:
> Another problem with PIO is support for physical virtio devices,
> and nested virt: KVM currently programs all PIO accesses
> to cause vm exit, so using this device in a VM will be slow.
Not answering your question, but support for programming direct PIO
access into KVM's I/O bitmap would be feasible. Such feature may have
some value for assigned devices that use PIO more heavily. They cause
lengthy user-space exists so far.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
Corporate Competence Center Embedded Linux

On Thu, Feb 28, 2013 at 05:24:33PM +0200, Michael S. Tsirkin
wrote:
> OK we talked about this a while ago, here's
> a summary and some proposals:
> At the moment, virtio PCI uses IO BARs for all accesses.
> 
> The reason for IO use is the cost of different VM exit types
> of transactions and their emulation on KVM on x86
> (it would be trivial to use memory BARs on non x86 platforms
>  if they don't have PIO).
> Example benchmark (cycles per transaction):
> 	(io access) outw 1737
> 	(memory access) movw 4341
> for comparison:
> 	(hypercall access): vmcall 1566
> 	(pv memory access) movw_fast 1817 (*explanation what this is below)
> 
> This creates a problem if we want to make virtio devices
> proper PCI express devices with native hotplug support.
> This is because each hotpluggable PCI express device always has
> a PCI express port (port per device),
> where each port is represented by a PCI to PCI bridge.
> In turn, a PCI to PCI bridge claims a 4Kbyte aligned
> range of IO addresses. This means that we can have at
> most 15 such devices, this is a nasty limitation.
> 
> Another problem with PIO is support for physical virtio devices,
> and nested virt: KVM currently programs all PIO accesses
> to cause vm exit, so using this device in a VM will be slow.
> 
> So we really want to stop using IO BARs completely if at all possible,
> but looking at the table above, switching to memory BAR and movw for
> notifications will not work well.
> 
> Possible solutions:
> 1. hypercall instead of PIO
> 	basically add a hypercall that gets an MMIO address/data
> 	and does an MMIO write for us.
> 	We'll want some capability in the device to let guest know
> 		this is what it should do.
> 	Pros: even faster than PIO
> 	Cons: this won't help nested or assigned devices (won't hurt
> 	      them either as it will be conditional on the capability above).
> 	Cons: need host kernel support, which then has to be maintained
> 	      forever, even if intel speeds up MMIO exits.
> 
> 2. pv memory access
> 	There are two reasons that memory access is slower:
> 		- one is that it's handled as an EPT misconfiguration error
> 		so handled by cpu slow path
> 		- one is that we need to decode the x86 instruction in
> 		software, to calculate address/data for the access.
> 
> 	We could agree that guests would use a specific instruction
> 	for virtio accesses, and fast-path it specifically.
> 	This is the pv memory access option above.
> 	Pros: helps assigned devices and nested virt
> 	Pros: easy to drop if hardware support is there
> 	Cons: a bit slower than IO
> 	Cons: need host kernel support
> 
> 3. hypervisor assigned IO address
> 	qemu can reserve IO addresses and assign to virtio devices.
> 	2 bytes per device (for notification and ISR access) will be
> 	enough. So we can reserve 4K and this gets us 2000 devices.
>         From KVM perspective, nothing changes.
> 	We'll want some capability in the device to let guest know
> 	this is what it should do, and pass the io address.
> 	One way to reserve the addresses is by using the bridge.
> 	Pros: no need for host kernel support
> 	Pros: regular PIO so fast
> 	Cons: does not help assigned devices, breaks nested virt
> 
> Simply counting pros/cons, option 3 seems best. It's also the
> easiest to implement.
Agree.

On 02/28/2013 07:24 AM, Michael S. Tsirkin wrote:
> 
> 3. hypervisor assigned IO address
> 	qemu can reserve IO addresses and assign to virtio devices.
> 	2 bytes per device (for notification and ISR access) will be
> 	enough. So we can reserve 4K and this gets us 2000 devices.
>         From KVM perspective, nothing changes.
> 	We'll want some capability in the device to let guest know
> 	this is what it should do, and pass the io address.
> 	One way to reserve the addresses is by using the bridge.
> 	Pros: no need for host kernel support
> 	Pros: regular PIO so fast
> 	Cons: does not help assigned devices, breaks nested virt
> 
> Simply counting pros/cons, option 3 seems best. It's also the
> easiest to implement.
> 
The problem here is the 4K I/O window for IO device BARs in bridges.
Why not simply add a (possibly proprietary) capability to the PCI bridge
to allow a much narrower window?  That fits much more nicely into the
device resource assignment on the guest side, and could even be
implemented on a real hardware device -- we can offer it to the PCI-SIG
for standardization, even.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

On Wed, Mar 06, 2013 at 03:15:16AM -0800, H. Peter Anvin
wrote:
> On 03/06/2013 01:21 AM, Michael S. Tsirkin wrote:
> > 
> > Right. Though even with better granularify bridge windows
> > would still be a (smaller) problem causing fragmentation.
> > 
> > If we were to extend the PCI spec I would go for a bridge without
> > windows at all: a bridge can snoop on configuration transactions and
> > responses programming devices behind it and build a full map of
address
> > to device mappings.
> > 
> > In partucular, this would be a good fit for an uplink bridge in a PCI
> > express switch, which is integrated with downlink bridges on the same
> > silicon, so bridge windows do nothing but add overhead.
> > 
> 
> True, but the real problem is that the downlink (type 1 header) is
> typically on a different piece of silicon than the device BAR (type 0
> header).
> 
> I am not sure that a snooping-based system will work and not be
> prohibitive in its hardware cost on an actual hardware system.  I
> suspect it would decay into needing a large RAM array in every bridge.
> 
> 	-hpa
This might be more difficult if you allow arbitrary number of functions
behind a bridge.  However, this is not a problem for integrated functions
(e.g. on the same silicon or board).

Bridge could specify which slots host integrated functions and do
snooping for these, and have regular window rules apply to open slots.


-- 
MST

On 02/28/2013 10:24 AM, Michael S. Tsirkin wrote:
> OK we talked about this a while ago, here's
> a summary and some proposals:
> At the moment, virtio PCI uses IO BARs for all accesses.
>
> The reason for IO use is the cost of different VM exit types
> of transactions and their emulation on KVM on x86
> (it would be trivial to use memory BARs on non x86 platforms
>   if they don't have PIO).
> Example benchmark (cycles per transaction):
> 	(io access) outw 1737
> 	(memory access) movw 4341
> for comparison:
> 	(hypercall access): vmcall 1566
> 	(pv memory access) movw_fast 1817 (*explanation what this is below)
>
> This creates a problem if we want to make virtio devices
> proper PCI express devices with native hotplug support.
> This is because each hotpluggable PCI express device always has
> a PCI express port (port per device),
> where each port is represented by a PCI to PCI bridge.
> In turn, a PCI to PCI bridge claims a 4Kbyte aligned
> range of IO addresses. This means that we can have at
> most 15 such devices, this is a nasty limitation.
>
> Another problem with PIO is support for physical virtio devices,
> and nested virt: KVM currently programs all PIO accesses
> to cause vm exit, so using this device in a VM will be slow.
>
> So we really want to stop using IO BARs completely if at all possible,
> but looking at the table above, switching to memory BAR and movw for
> notifications will not work well.
>
> Possible solutions:
> 1. hypercall instead of PIO
> 	basically add a hypercall that gets an MMIO address/data
> 	and does an MMIO write for us.
> 	We'll want some capability in the device to let guest know
> 		this is what it should do.
> 	Pros: even faster than PIO
> 	Cons: this won't help nested or assigned devices (won't hurt
> 	      them either as it will be conditional on the capability above).
> 	Cons: need host kernel support, which then has to be maintained
> 	      forever, even if intel speeds up MMIO exits.
>
> 2. pv memory access
> 	There are two reasons that memory access is slower:
> 		- one is that it's handled as an EPT misconfiguration error
> 		so handled by cpu slow path
> 		- one is that we need to decode the x86 instruction in
> 		software, to calculate address/data for the access.
>
> 	We could agree that guests would use a specific instruction
> 	for virtio accesses, and fast-path it specifically.
> 	This is the pv memory access option above.
> 	Pros: helps assigned devices and nested virt
> 	Pros: easy to drop if hardware support is there
> 	Cons: a bit slower than IO
> 	Cons: need host kernel support
>
> 3. hypervisor assigned IO address
> 	qemu can reserve IO addresses and assign to virtio devices.
> 	2 bytes per device (for notification and ISR access) will be
> 	enough. So we can reserve 4K and this gets us 2000 devices.
>          From KVM perspective, nothing changes.
> 	We'll want some capability in the device to let guest know
> 	this is what it should do, and pass the io address.
> 	One way to reserve the addresses is by using the bridge.
> 	Pros: no need for host kernel support
> 	Pros: regular PIO so fast
> 	Cons: does not help assigned devices, breaks nested virt
>
> Simply counting pros/cons, option 3 seems best. It's also the
> easiest to implement.
>
> Comments?
>
apologies for late response...

It seems that solution 1 would be the best option for the following reasons:

a) (nearly?) every virt technology out there (xen, kvm, vmware, hyperv)
    has pv drivers in the major OS's using virt (Windows, Linux),
    so having a hypercall table searched, initialized and used for
    fast virtio register access is trivially simple to do.

b) the support can be added with whatever pvdriver set is provided
    w/o impacting OS core support.

c) it's architecture neutral, or can be made architecture neutral.
    e.g., inb/outb & PCI ioport support is very different btwn x86 &
non-x86.
    A hypercall interface would not have that dependency/difference.
  
d) it doesn't require new OS support in std/core areas for
    new standard(s), as another thread proposed; this kind of approach
    has a long, time delay to get defined & implemented across OS's.
    In contrast, a hypercall defined interface can be indep. of standards
    bodies, and if built into a pvdriver core, can change &/or adapt
rapidly,
    and have additional i/f mechanisms for version-levels, which enables
    cross-hypervisor(version) migration.

e) the hypercall can be extended to do pv-specific hot add/remove,
    eliminating dependencies on emulation support of ACPI-hp or PCIe-hp,
    and simply(?) track core interfaces for hot-plug of (class) devices.

f) For migration, hypercall interfaces could be extended for better/faster
    migration as well (suspend/resume pv device).

my (late) 5 cents (I'll admit it was more than 2 cents)... Don