Re: [Xen-devel] [PATCH] Ignore non-zero data in unused xsave area.

[Don Koch <dkoch@xxxxxxxxxxx>]

This change can be ignored. Details below.

On Tue, 18 Nov 2014 10:26:31 -0500
Don Koch <dkoch@xxxxxxxxxxx> wrote:

> If we restore an xsave area from an older xen that has a larger
> size than the xcr0 bit call for, it is possible to have non-zero
> data in the unused area if an xsave has ever been done that used
> that area (e.g. during a context switch). Since the vcpu's xsave
> area is never zeroed after the initial allocation, that data is
> still there. Since we are told that said area was not written to
> during the save or migration, there is no need to restore it.

Found the issue. Don Slutz had reported this occurring a couple of times
and I witnessed the bad data in the unused area. Turns out he had
done a migration from 4.3 to 4.4, then another migration from 4.4 to
4.3 and when he tried another 4.3 to 4.4 migration, it failed.

Since 4.4 to 4.3 migration isn't supported, we can punt this fix.
Well, unless we want to support this (I assume not) in which case
it still needs a minor tweak.

Sorry for the noise.

-d

> Signed-off-by: Don Koch <dkoch@xxxxxxxxxxx>
> ---
> Turns out the assertion that the unused xsave area is zero
> is wrong. Unfortunately, that leaves the following as the
> only way I can think of to work around it (and is no worse
> than xsave/xrestore during context switches). Alternate
> suggestions welcome.
> 
>  xen/arch/x86/hvm/hvm.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index 8f49b44..b2c0bc4 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -2044,7 +2044,7 @@ static int hvm_load_cpu_xsave_states(struct domain *d, 
> hvm_domain_context_t *h)
>                  printk(XENLOG_G_WARNING
>                         "HVM%d.%u restore mismatch: xsave length %#x > %#x 
> (non-zero data at %#x)\n",
>                         d->domain_id, vcpuid, desc->length, size, i);
> -                return -EOPNOTSUPP;
> +                break;
>              }
>          }
>          printk(XENLOG_G_WARNING
> -- 
> 1.8.3.1
> 

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