From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
kvm@vger.kernel.org, x86@kernel.org,
Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>, Gavin Shan <gshan@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/8] KVM: x86: extend struct kvm_vcpu_pv_apf_data with token info
Date: Fri, 15 May 2020 13:53:24 -0700 [thread overview]
Message-ID: <20200515205323.GG17572@linux.intel.com> (raw)
In-Reply-To: <20200515203352.GC235744@redhat.com>
On Fri, May 15, 2020 at 04:33:52PM -0400, Vivek Goyal wrote:
> On Fri, May 15, 2020 at 09:18:07PM +0200, Paolo Bonzini wrote:
> > On 15/05/20 20:46, Sean Christopherson wrote:
> > >> The new one using #VE is not coming very soon (we need to emulate it for
> > >> <Broadwell and AMD processors, so it's not entirely trivial) so we are
> > >> going to keep "page not ready" delivery using #PF for some time or even
> > >> forever. However, page ready notification as #PF is going away for good.
> > >
> > > And isn't hardware based EPT Violation #VE going to require a completely
> > > different protocol than what is implemented today? For hardware based #VE,
> > > KVM won't intercept the fault, i.e. the guest will need to make an explicit
> > > hypercall to request the page.
> >
> > Yes, but it's a fairly simple hypercall to implement.
> >
> > >> That said, type1/type2 is quite bad. :) Let's change that to page not
> > >> present / page ready.
> > >
> > > Why even bother using 'struct kvm_vcpu_pv_apf_data' for the #PF case? VMX
> > > only requires error_code[31:16]==0 and SVM doesn't vet it at all, i.e. we
> > > can (ab)use the error code to indicate an async #PF by setting it to an
> > > impossible value, e.g. 0xaaaa (a is for async!). That partciular error code
> > > is even enforced by the SDM, which states:
> >
> > Possibly, but it's water under the bridge now.
> > And the #PF mechanism also has the problem with NMIs that happen before
> > the error code is read
> > and page faults happening in the handler (you may connect some dots now).
>
> I understood that following was racy.
>
> do_async_page_fault <--- kvm injected async page fault
> NMI happens (Before kvm_read_and_reset_pf_reason() is done)
> ->do_async_page_fault() (This is regular page fault but it will read
> reason from shared area and will treat itself
> as async page fault)
>
> So this is racy.
>
> But if we get rid of the notion of reading from shared region in page
> fault handler, will we not get rid of this race.
>
> I am assuming that error_code is not racy as it is pushed on stack.
> What am I missing.
Nothing, AFAICT. As I mentioned in a different mail, CR2 can be squished,
but I don't see how error code can be lost.
But, because CR2 can be squished, there still needs to be an in-memory busy
flag even if error code is used as the host #PF indicator, otherwise the
guest could lose one of the tokens.
next prev parent reply other threads:[~2020-05-15 20:53 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-11 16:47 [PATCH 0/8] KVM: x86: Interrupt-based mechanism for async_pf 'page present' notifications Vitaly Kuznetsov
2020-05-11 16:47 ` [PATCH 1/8] Revert "KVM: async_pf: Fix #DF due to inject "Page not Present" and "Page Ready" exceptions simultaneously" Vitaly Kuznetsov
2020-05-11 16:47 ` [PATCH 2/8] KVM: x86: extend struct kvm_vcpu_pv_apf_data with token info Vitaly Kuznetsov
2020-05-12 15:27 ` Vivek Goyal
2020-05-12 15:40 ` Vitaly Kuznetsov
2020-05-12 15:53 ` Vivek Goyal
2020-05-12 17:50 ` Sean Christopherson
2020-05-13 9:09 ` Vitaly Kuznetsov
2020-05-13 12:52 ` Vivek Goyal
2020-05-15 15:59 ` Paolo Bonzini
2020-05-15 18:46 ` Sean Christopherson
2020-05-15 19:18 ` Paolo Bonzini
2020-05-15 20:33 ` Vivek Goyal
2020-05-15 20:53 ` Sean Christopherson [this message]
2020-05-15 20:43 ` Sean Christopherson
2020-05-15 22:23 ` Paolo Bonzini
2020-05-15 23:16 ` Sean Christopherson
2020-05-21 14:59 ` Vitaly Kuznetsov
2020-05-22 7:33 ` Paolo Bonzini
2020-05-12 21:15 ` Vivek Goyal
2020-05-21 18:38 ` Vivek Goyal
2020-05-23 16:34 ` Vitaly Kuznetsov
2020-05-26 12:50 ` Vivek Goyal
2020-05-11 16:47 ` [PATCH 3/8] KVM: introduce kvm_read_guest_offset_cached() Vitaly Kuznetsov
2020-05-11 16:47 ` [PATCH 4/8] KVM: x86: interrupt based APF page-ready event delivery Vitaly Kuznetsov
2020-05-12 14:24 ` Vivek Goyal
2020-05-12 15:50 ` Vitaly Kuznetsov
2020-05-12 18:07 ` Vivek Goyal
2020-05-13 9:03 ` Vitaly Kuznetsov
2020-05-13 13:53 ` Vivek Goyal
2020-05-13 14:03 ` Vivek Goyal
2020-05-13 14:23 ` Vitaly Kuznetsov
2020-05-13 18:46 ` Vivek Goyal
2020-05-14 8:08 ` Vitaly Kuznetsov
2020-05-14 13:31 ` Vivek Goyal
2020-05-11 16:47 ` [PATCH 5/8] KVM: x86: acknowledgment mechanism for async pf page ready notifications Vitaly Kuznetsov
2020-05-11 16:47 ` [PATCH 6/8] KVM: x86: announce KVM_FEATURE_ASYNC_PF_INT Vitaly Kuznetsov
2020-05-11 16:47 ` [PATCH 7/8] KVM: x86: Switch KVM guest to using interrupts for page ready APF delivery Vitaly Kuznetsov
2020-05-11 16:47 ` [PATCH 8/8] KVM: x86: drop KVM_PV_REASON_PAGE_READY case from kvm_handle_page_fault() Vitaly Kuznetsov
2020-05-12 15:32 ` [PATCH 0/8] KVM: x86: Interrupt-based mechanism for async_pf 'page present' notifications Vivek Goyal
2020-05-12 16:12 ` Vitaly Kuznetsov
2020-05-13 14:16 ` Vivek Goyal
2020-05-14 18:14 ` Vitaly Kuznetsov
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