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

[Boris Ostrovsky]

On 04/30/2015 03:35 PM, Andy Lutomirski wrote:
> On Thu, Apr 30, 2015 at 12:30 PM, Boris Ostrovsky
> <boris.ostrovsky@xxxxxxxxxx> wrote:
> > 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.
> http://article.gmane.org/gmane.linux.kernel/1937899
>
> The change was that we stopped reloading ss after 32-bit sysret in the
> vdso trampoline.  Before we always reloaded ss in userspace before
> doing any stack operations.
>
> I have no idea whether this was deliberate.  It had done that (with
> the attendant performance hit) since the very beginning, and there was
> no comment explaining why.

The test passed. Which I guess is not surprising given that interrupt 
handling is done by the hypervisor.


-boris
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