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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, May 21, 2020 at 10:43:58PM +0100, 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.

Couldn't direct MMIO regions be handled like other types of memory for
the purposes of logdiry mode?

I assume there's already a path here used for other memory types when
logdirty is turned on, and hence would seem better to just make direct
MMIO regions also use that path?

> Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
> ---
>  xen/arch/x86/hvm/svm/svm.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
> index 46a1aac..f0d0bd3 100644
> --- a/xen/arch/x86/hvm/svm/svm.c
> +++ b/xen/arch/x86/hvm/svm/svm.c
> @@ -1726,6 +1726,10 @@ static void svm_do_nested_pgfault(struct vcpu *v,
>          /* inject #VMEXIT(NPF) into guest. */
>          nestedsvm_vmexit_defer(v, VMEXIT_NPF, pfec, gpa);
>          return;
> +    case 0:
> +        /* If a recalculation page fault hasn't been handled - just retry. */
> +        if ( pfec & PFEC_user_mode )
> +            return;

I'm slightly worried that this diverges from the EPT implementation
now, in the sense that returning 0 from hvm_hap_nested_page_fault will
no longer trigger a guest crash.

Thanks, Roger.