Xen devel - Jul 2005 - [PATCH] Fix debug exception stack to enable gdb on xeno64

Native x86_64 linux uses paranoidentry for exceptions with ist != 0,
while x86_64 xenlinux doesn''t have special stacks for these exceptions.
Actually we don''t need paranoidentry for x86_64 xenlinux, and these
exceptions except #DB should be handled in hypervisor.  With this patch,
#DB is forwarded to guest, and let it use zeroentry should be OK.
-Xin

Signed-off-by: Xin Li <xin.b.li@intel.com>






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"Li, Xin B" <xin.b.li@intel.com> writes:
> Native x86_64 linux uses paranoidentry for exceptions with ist != 0,
> while x86_64 xenlinux doesn''t have special stacks for these
exceptions.
> Actually we don''t need paranoidentry for x86_64 xenlinux, and
these
> exceptions except #DB should be handled in hypervisor.  With this patch,
> #DB is forwarded to guest, and let it use zeroentry should be OK.
> -Xin
Doesn''t look correct to me. When you get an NMI or DB directly after 
a SYSCALL to Xen it''ll execute on a random stack and likely break
if the guest runs with a broken stack at that time.

Also I can really recommend having IST stacks for double fault and stack
fault, otherwise everytime you break the stack you get a triple fault
which are nasty to debug. You guys with target probes might not
care that much, but others do.

-Andi

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On 12 Jul 2005, at 18:10, Andi Kleen wrote:
> Doesn''t look correct to me. When you get an NMI or DB directly
after
> a SYSCALL to Xen it''ll execute on a random stack and likely break
> if the guest runs with a broken stack at that time.
>
> Also I can really recommend having IST stacks for double fault and 
> stack
> fault, otherwise everytime you break the stack you get a triple fault
> which are nasty to debug. You guys with target probes might not
> care that much, but others do.
The patch is for guest Linux. Xen itself does use IST stacks, just like 
native Linux.

  -- Keir


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On Tue, Jul 12, 2005 at 06:21:59PM +0100, Keir Fraser
wrote:
> The patch is for guest Linux. Xen itself does use IST stacks, just like 
> native Linux.
Same issue then, just from user space.

-Andi

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