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 12:30 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.

--Andy

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