linux-kernel - Re: [PATCH RESEND v4 7/9] KVM: VMX: Handle SPP induced vmexit and page fault

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Date:   Tue, 20 Aug 2019 21:44:35 +0800
From:   Yang Weijiang <weijiang.yang@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Yang Weijiang <weijiang.yang@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, sean.j.christopherson@...el.com,
        mst@...hat.com, rkrcmar@...hat.com, jmattson@...gle.com,
        yu.c.zhang@...el.com, alazar@...defender.com
Subject: Re: [PATCH RESEND v4 7/9] KVM: VMX: Handle SPP induced vmexit and
 page fault

On Mon, Aug 19, 2019 at 05:04:23PM +0200, Paolo Bonzini wrote:
> On 19/08/19 16:43, Paolo Bonzini wrote:
> >> +			/*
> >> +			 * Record write protect fault caused by
> >> +			 * Sub-page Protection, let VMI decide
> >> +			 * the next step.
> >> +			 */
> >> +			if (spte & PT_SPP_MASK) {
> > Should this be "if (spte & PT_WRITABLE_MASK)" instead?  That is, if the
> > page is already writable, the fault must be an SPP fault.
> 
> Hmm, no I forgot how SPP works; still, this is *not* correct.  For
> example, if SPP marks part of a page as read-write, but KVM wants to
> write-protect the whole page for access or dirty tracking, that should
> not cause an SPP exit.
> 
> So I think that when KVM wants to write-protect the whole page
> (wrprot_ad_disabled_spte) it must also clear PT_SPP_MASK; for example it
> could save it in bit 53 (PT64_SECOND_AVAIL_BITS_SHIFT + 1).  If the
> saved bit is set, fast_page_fault must then set PT_SPP_MASK instead of
> PT_WRITABLE_MASK.
Sure, will change the processing flow.

> On re-entry this will cause an SPP vmexit;
> fast_page_fault should never trigger an SPP userspace exit on its own,
> all the SPP handling should go through handle_spp.
> 
> Paolo