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

  • To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Date: Fri, 22 May 2020 15:32:59 +0200
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  • Cc: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>, wl@xxxxxxx, Jan Beulich <jbeulich@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
  • Delivery-date: Fri, 22 May 2020 13:33:11 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On Fri, May 22, 2020 at 02:11:15PM +0100, Andrew Cooper wrote:
> On 22/05/2020 14:04, Jan Beulich wrote:
> > On 22.05.2020 13:11, Roger Pau Monné wrote:
> >> That being said, I also don't like the fact that logdity is handled
> >> differently between EPT and NPT, as on EPT it's handled as a
> >> misconfig while on NPT it's handled as a violation.
> > Because, well, there is no concept of misconfig in NPT.
> Indeed.  Intel chose to split EPT errors into two - MISCONFIG for
> structural errors (not present, or reserved bits set) and VIOLATION for
> permissions errors.
> AMD reused the same silicon pagewalker design, so have a single
> NPT_FAULT vmexit which behaves much more like a regular pagefault,
> encoding structural vs permission errors in the error code.

Maybe I should clarify, I understand that NPT doesn't have such
differentiation regarding nested page table faults vs EPT, but I feel
like it would be clearer if part of the code could be shared, ie:
unify EPT resolve_misconfig and NPT do_recalc into a single function
for example that uses the necessary p2m-> helpers for the differing
implementations. I think we should be able to tell apart when a NPT
page fault is a recalc one by looking at the bits in the EXITINFO1
error field?

Anyway, this was just a rant, and it's tangential to the issue at
hand, sorry for distracting.




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