Re: [PATCH] x86/svm: retry after unhandled NPT fault if gfn was marked for recalculation

[Igor Druzhinin <igor.druzhinin@citrix.com>]

On 21/05/2020 22:43, Igor Druzhinin wrote:
> If a recalculation NPT fault hasn't been handled explicitly in
> hvm_hap_nested_page_fault() then it's potentially safe to retry -
> US bit has been re-instated in PTE and any real fault would be correctly
> re-raised next time.
> 
> This covers a specific case of migration with vGPU assigned on AMD:
> global log-dirty is enabled and causes immediate recalculation NPT
> fault in MMIO area upon access. This type of fault isn't described
> explicitly in hvm_hap_nested_page_fault (this isn't called on
> EPT misconfig exit on Intel) which results in domain crash.
> 
> Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
> ---

Alternatively, I can re-raise the fault immediately after recalculation is
done which is less efficient (will take one more VMEXIT) but safer IMO -
hvm_hap_nested_page_fault might potentially leave VM in inconsistent state
in case of a real failure and cause second page fault to conceal it.

Another alternative is to inject fall_through bool into hvm_hap_nested_page_fault
to give it the idea of expected behavior in that case and avoid guessing in SVM
code. I think that's an improvement over suggestion in v1 and a candidate for v2. 

Igor