From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org
Cc: "Radim Krčmář" <rkrcmar@redhat.com>,
"Junaid Shahid" <junaids@google.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/kvm/mmu: make mmu->prev_roots cache work for NPT case
Date: Fri, 22 Feb 2019 19:49:35 +0100 [thread overview]
Message-ID: <87tvgvslog.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <bb45499b-bdb2-f170-74ee-09c4eaf41e1a@redhat.com>
Paolo Bonzini <pbonzini@redhat.com> writes:
> On 22/02/19 17:46, Vitaly Kuznetsov wrote:
>> I noticed that fast_cr3_switch() always fails when we switch back from L2
>> to L1 as it is not able to find a cached root. This is odd: host's CR3
>> usually stays the same, we expect to always follow the fast path. Turns
>> out the problem is that page role is always mismatched because
>> kvm_mmu_get_page() filters out cr4_pae when direct, the value is stored
>> in page header and later compared with new_role in cached_root_available().
>> As cr4_pae is always set in long mode prev_roots cache is dysfunctional.
>
> Really cr4_pae means "are the PTEs 8 bytes". So I think your patch is
> correct but on top we should set it to 1 (not zero!!) for
> kvm_calc_shadow_ept_root_page_role, init_kvm_nested_mmu and
> kvm_calc_tdp_mmu_root_page_role. Or maybe everything breaks with that
> change.
>
Yes, exactly. If we put '1' there kvm_mmu_get_page() will again filter
it out and we won't be able to find the root in prev_roots cache :-(
>> - Do not clear cr4_pae in kvm_mmu_get_page() and check direct on call sites
>> (detect_write_misaligned(), get_written_sptes()).
>
> These only run with shadow page tables, by the way.
>
Yes, and that's why I think it may make sense to move the filtering
logic there. At least in other cases cr4_pae will always be equal to
is_pae().
It seems I know too little about shadow paging and all these corner
cases :-(
--
Vitaly
next prev parent reply other threads:[~2019-02-22 18:49 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-22 16:46 [PATCH] x86/kvm/mmu: make mmu->prev_roots cache work for NPT case Vitaly Kuznetsov
2019-02-22 17:17 ` Paolo Bonzini
2019-02-22 18:49 ` Vitaly Kuznetsov [this message]
2019-02-22 20:29 ` Paolo Bonzini
2019-02-23 11:15 ` [PATCH v2 RFC] " Vitaly Kuznetsov
2019-03-07 14:07 ` Vitaly Kuznetsov
2019-03-07 16:06 ` Sean Christopherson
2019-03-07 16:41 ` Vitaly Kuznetsov
2019-03-07 18:59 ` Sean Christopherson
2019-03-07 19:02 ` Sean Christopherson
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