Re: [Xen-devel] [PATCH] hypervisor/x86/xen: Unset X86_BUG_SYSRET_SS_ATTRS on Xen PV guests

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Thu, Apr 30, 2015 at 12:08 PM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> wrote:
> Commit 61f01dd941ba ("x86_64, asm: Work around AMD SYSRET SS descriptor
> attribute issue") makes AMD processors set SS to __KERNEL_DS in
> __switch_to() to deal with cases when SS is NULL.
>
> This breaks Xen PV guests who do not want to load SS with__KERNEL_DS.
>
> Since the problem that the commit is trying to address would have to be
> fixed in the hypervisor (if it in fact exists under Xen) there is no
> reason to set X86_BUG_SYSRET_SS_ATTRS flag for PV VPCUs here.
>

Seems reasonable.

Have you run the test case on a Xen PV guest on AMD?  It's possible
that Xen is affected, since the old accidental workaround that we
deleted was in the vdso and probably would have worked on Xen.

--Andy

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