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

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Date:   Thu, 22 Aug 2019 21:17:45 +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 Tue, Aug 20, 2019 at 09:44:35PM +0800, Yang Weijiang wrote:
> 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.
 Hi, Paolo,
 According to the latest SDM(28.2.4), handle_spp only handles SPPT miss and SPPT
 misconfig(exit_reason==66), subpage write access violation causes EPT violation,
 so have to deal with the two cases into handlers.
> > Paolo