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

[Boris Ostrovsky]

On 04/30/2015 03:17 PM, Andy Lutomirski wrote:
> 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.

Is there a specific test that would trigger this bug? I 
booted/suspended/resumed a bunch of various guest types on both AMD and 
Intel but not much more than that. What is the commit that you deleted?

I doubt that a change in the guest could suddenly start triggering this 
issue but I will take a look at hypervisor code.


-boris

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