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

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Message-ID: <fb6cd8b4-eee9-6e58-4047-550811bffd58@redhat.com>
Date:   Mon, 19 Aug 2019 17:04:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yang Weijiang <weijiang.yang@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, sean.j.christopherson@...el.com
Cc:     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 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.  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