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SubjectRe: [PATCH] x86/mm: Don't try to change poison pages to uncacheable in a guest
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There is only one actual machine check. But the VMM simulates a second machine check to the guest when the guest tries to access the poisoned page.

The stack trace was from Jue. I didn’t try to check it. But it looked reasonable that Linux would flush the cache for a page that is transitioning from cacheable to uncacheable.

Sent from my iPhone

> On May 15, 2020, at 23:54, Borislav Petkov <bp@alien8.de> wrote:
>
> On Tue, May 05, 2020 at 11:46:48AM -0700, Tony Luck wrote:
>> An interesting thing happened when a guest Linux instance took
>> a machine check. The VMM unmapped the bad page from guest physical
>> space and passed the machine check to the guest.
>>
>> Linux took all the normal actions to offline the page from the process
>> that was using it. But then guest Linux crashed because it said there
>> was a second machine check inside the kernel with this stack trace:
>>
>> do_memory_failure
>> set_mce_nospec
>> set_memory_uc
>> _set_memory_uc
>> change_page_attr_set_clr
>> cpa_flush
>> clflush_cache_range_opt
>
> Maybe I don't see it but how can clflush_cache_range_opt() call
> cpa_flush() ?
>
>> This was odd, because a CLFLUSH instruction shouldn't raise a machine
>> check (it isn't consuming the data). Further investigation showed that
>> the VMM had passed in another machine check because is appeared that the
>> guest was accessing the bad page.
>
> This is where you lost me - if the VMM unmaps the page during the first
> MCE, how can the guest even attempt to touch it and do this stack trace
> above?
>
> /me is confused.
>
> --
> Regards/Gruss,
> Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette
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