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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 04/30/2015 03:35 PM, Andy Lutomirski wrote:
> On Thu, Apr 30, 2015 at 12:30 PM, Boris Ostrovsky
> <boris.ostrovsky@oracle.com> wrote:
>> On 04/30/2015 03:17 PM, Andy Lutomirski wrote:
>>> On Thu, Apr 30, 2015 at 12:08 PM, Boris Ostrovsky
>>> <boris.ostrovsky@oracle.com> 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