Andy Lutomirski
2014-Sep-18 15:38 UTC
Standardizing an MSR or other hypercall to get an RNG seed?
On Thu, Sep 18, 2014 at 7:43 AM, H. Peter Anvin <hpa at zytor.com> wrote:> On 09/18/2014 07:40 AM, KY Srinivasan wrote: >>> >>> The main questions are what MSR index to use and how to detect the >>> presence of the MSR. I've played with two approaches: >>> >>> 1. Use CPUID to detect the presence of this feature. This is very easy for >>> KVM to implement by using a KVM-specific CPUID feature. The problem is >>> that this will necessarily be KVM-specific, as the guest must first probe for >>> KVM and then probe for the KVM feature. I doubt that Hyper-V, for >>> example, wants to claim to be KVM. If we could standardize a non- >>> hypervisor-specific CPUID feature, then this problem would go away. >> >> We would prefer a CPUID feature bit to detect this feature. >> > > I guess if we're introducing the concept of pan-OS MSRs we could also > have pan-OS CPUID. The real issue is to get a single non-conflicting > standard.Agreed. KVM currently puts 0 in 0x40000000.EAX, meaning that a feature bit in Microsoft's leaf 0x40000003 would probably not work well for KVM. I don't expect that Microsoft wants to start claiming to be KVM for the purpose of using a KVM-style feature bit, so, if we went the CPUID route, we would probably need something new. --Andy> > -hpa > >-- Andy Lutomirski AMA Capital Management, LLC
Andy Lutomirski
2014-Sep-18 15:44 UTC
Standardizing an MSR or other hypercall to get an RNG seed?
On Thu, Sep 18, 2014 at 8:38 AM, Andy Lutomirski <luto at amacapital.net> wrote:> On Thu, Sep 18, 2014 at 7:43 AM, H. Peter Anvin <hpa at zytor.com> wrote: >> On 09/18/2014 07:40 AM, KY Srinivasan wrote: >>>> >>>> The main questions are what MSR index to use and how to detect the >>>> presence of the MSR. I've played with two approaches: >>>> >>>> 1. Use CPUID to detect the presence of this feature. This is very easy for >>>> KVM to implement by using a KVM-specific CPUID feature. The problem is >>>> that this will necessarily be KVM-specific, as the guest must first probe for >>>> KVM and then probe for the KVM feature. I doubt that Hyper-V, for >>>> example, wants to claim to be KVM. If we could standardize a non- >>>> hypervisor-specific CPUID feature, then this problem would go away. >>> >>> We would prefer a CPUID feature bit to detect this feature. >>> >> >> I guess if we're introducing the concept of pan-OS MSRs we could also >> have pan-OS CPUID. The real issue is to get a single non-conflicting >> standard. > > Agreed. > > KVM currently puts 0 in 0x40000000.EAX, meaning that a feature bit in > Microsoft's leaf 0x40000003 would probably not work well for KVM. I > don't expect that Microsoft wants to start claiming to be KVM for the > purpose of using a KVM-style feature bit, so, if we went the CPUID > route, we would probably need something new.Slight correction: QEMU/KVM has optional support for Hyper-V feature enumeration. Ideally the RNG seed mechanism would be enabled by default, but I don't know whether the QEMU maintainers would be okay with enabling the Hyper-V cpuid mechanism in a default configuration. --Andy> > --Andy > >> >> -hpa >> >> > > > > -- > Andy Lutomirski > AMA Capital Management, LLC-- Andy Lutomirski AMA Capital Management, LLC
Paolo Bonzini
2014-Sep-18 15:58 UTC
Standardizing an MSR or other hypercall to get an RNG seed?
Il 18/09/2014 17:44, Andy Lutomirski ha scritto:> Slight correction: QEMU/KVM has optional support for Hyper-V feature > enumeration. Ideally the RNG seed mechanism would be enabled by > default, but I don't know whether the QEMU maintainers would be okay > with enabling the Hyper-V cpuid mechanism in a default configuration.Some guests cannot find the KVM leaves at 0x40000100, so it wouldn't be great. And I also don't know what VMware folks would think, but I think they would be even less thrilled than me. Note that even if there is no well-defined CPUID leaf, and the main detection mechanism is #GP, each hypervisor is free to define a CPUID bit of its own. However, if it's going to be an architectural (Intel-defined) MSR, I think the right place for a feature bit is in the low leaves (like EAX=7, ECX=0). Paolo
KY Srinivasan
2014-Sep-18 16:36 UTC
Standardizing an MSR or other hypercall to get an RNG seed?
> -----Original Message----- > From: Andy Lutomirski [mailto:luto at amacapital.net] > Sent: Thursday, September 18, 2014 8:38 AM > To: H. Peter Anvin > Cc: KY Srinivasan; Linux Virtualization; kvm list; Gleb Natapov; Paolo Bonzini; > Theodore Ts'o > Subject: Re: Standardizing an MSR or other hypercall to get an RNG seed? > > On Thu, Sep 18, 2014 at 7:43 AM, H. Peter Anvin <hpa at zytor.com> wrote: > > On 09/18/2014 07:40 AM, KY Srinivasan wrote: > >>> > >>> The main questions are what MSR index to use and how to detect the > >>> presence of the MSR. I've played with two approaches: > >>> > >>> 1. Use CPUID to detect the presence of this feature. This is very > >>> easy for KVM to implement by using a KVM-specific CPUID feature. > >>> The problem is that this will necessarily be KVM-specific, as the > >>> guest must first probe for KVM and then probe for the KVM feature. > >>> I doubt that Hyper-V, for example, wants to claim to be KVM. If we > >>> could standardize a non- hypervisor-specific CPUID feature, then this > problem would go away. > >> > >> We would prefer a CPUID feature bit to detect this feature. > >> > > > > I guess if we're introducing the concept of pan-OS MSRs we could also > > have pan-OS CPUID. The real issue is to get a single non-conflicting > > standard. > > Agreed. > > KVM currently puts 0 in 0x40000000.EAX, meaning that a feature bit in > Microsoft's leaf 0x40000003 would probably not work well for KVM. I don't > expect that Microsoft wants to start claiming to be KVM for the purpose of > using a KVM-style feature bit, so, if we went the CPUID route, we would > probably need something new. > > --AndyI am copying other Hyper-V engineers to this discussion. Regards, K. Y> > > > > -hpa > > > > > > > > -- > Andy Lutomirski > AMA Capital Management, LLC
Nakajima, Jun
2014-Sep-18 17:13 UTC
Standardizing an MSR or other hypercall to get an RNG seed?
On Thu, Sep 18, 2014 at 9:36 AM, KY Srinivasan <kys at microsoft.com> wrote:> > I am copying other Hyper-V engineers to this discussion. >Thanks, K.Y. In terms of the address for the MSR, I suggest that you choose one from the range between 40000000H - 400000FFH. The SDM (35.1 ARCHITECTURAL MSRS) says "All existing and future processors will not implement any features using any MSR in this range." Hyper-V already defines many synthetic MSRs in this range, and I think it would be reasonable for you to pick one for this to avoid a conflict? -- Jun Intel Open Source Technology Center
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- Standardizing an MSR or other hypercall to get an RNG seed?
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