On 12/09/18 11:05, Jan Beulich wrote:
>>>> On 12.09.18 at 11:24, <andrew.coop...@citrix.com> wrote:
>> On 12/09/18 10:17, Jan Beulich wrote:
>>>>>> On 12.09.18 at 11:12, <andrew.coop...@citrix.com> wrote:
>>>> On 12/09/18 09:29, Sergey Dyasli wrote:
>>>>> On Tue, 2018-09-11 at 19:56 +0100, Andrew Cooper wrote:
>>>>>> @@ -822,13 +818,13 @@ int wrmsr_hypervisor_regs(uint32_t idx, uint64_t
>>>>>> val)
>>>>>> if ( p2m_is_paging(t) )
>>>>>> {
>>>>>> p2m_mem_paging_populate(d, gmfn);
>>>>>> - return -ERESTART;
>>>>>> + return X86EMUL_RETRY;
>>>>> Previously -ERESTART would've been converted to X86EMUL_EXCEPTION. But
>>>>> with this patch, X86EMUL_RETRY will actually be returned. I don't think
>>>>> that callers can handle this situation.
>>>>>
>>>>> E.g. the code from vmx_vmexit_handler():
>>>>>
>>>>> case EXIT_REASON_MSR_WRITE:
>>>>> switch ( hvm_msr_write_intercept(regs->ecx, msr_fold(regs), 1) )
>>>>> {
>>>>> case X86EMUL_OKAY:
>>>>> update_guest_eip(); /* Safe: WRMSR */
>>>>> break;
>>>>>
>>>>> case X86EMUL_EXCEPTION:
>>>>> hvm_inject_hw_exception(TRAP_gp_fault, 0);
>>>>> break;
>>>>> }
>>>>> break;
>>>> Hmm lovely, so it was broken before, but should be correct now.
>>>>
>>>> RETRY has caused an entry to go onto the paging ring, which will pause
>>>> the vcpu until a reply occurs, after which we will re-enter the guest
>>>> without having moved RIP forwards, re-execute the wrmsr instruction, and
>>>> this time succeed because the frame has been paged in.
>>> But then perhaps split out the actual bugfix into a prereq patch,
>>> especially as that one may want backporting?
>> Sorry, but I'm not sure you can't have that. It will depend on the
>> internals of the hooks, but I suspect you'd need the entire rest of my
>> MSR cleanup work if you wanted to cleanly backport this bugfix.
> Ouch, well - then: No.
Turns out it was a transitory bug introduced by patch 1, so no cause for
alarm.
It is latently buggy on the PV side, but there is no paging support for
PV guests yet, so no need to backport any fixes.
~Andrew
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