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Re: [Xen-devel] [PATCH 1/2] x86/vtx: Fix fault semantics for early task switch failures


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Fri, 22 Nov 2019 13:12:22 +0000
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Fri, 22 Nov 2019 13:12:30 +0000
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On 22/11/2019 13:08, Jan Beulich wrote:
> On 22.11.2019 13:37, Roger Pau Monné  wrote:
>> On Thu, Nov 21, 2019 at 10:15:50PM +0000, Andrew Cooper wrote:
>>> The VT-x task switch handler adds inst_len to rip before calling
>>> hvm_task_switch().  This causes early faults to be delivered to the guest 
>>> with
>>> trap semantics, and break restartibility.
>>>
>>> Instead, pass the instruction length into hvm_task_switch() and write it 
>>> into
>>> the outgoing tss only, leaving rip in its original location.
>>>
>>> For now, pass 0 on the SVM side.  This highlights a separate preexisting bug
>>> which will be addressed in the following patch.
>>>
>>> While adjusting call sites, drop the unnecessary uint16_t cast.
>>>
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Code LGTM:
>>
>> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

It occurs to me that this also fixes a vmentry failure in the corner
case that an instruction, which crosses the 4G=>0 boundary takes a
fault.  %rip will be adjusted without being truncated.

~Andrew

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