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[91.219.240.2]) by smtp.gmail.com with ESMTPSA id t13sm1588514wmj.15.2020.10.05.01.42.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Oct 2020 01:42:37 -0700 (PDT) From: Vitaly Kuznetsov To: kvm@vger.kernel.org, x86@kernel.org Cc: Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Peter Zijlstra , Paolo Bonzini , linux-kernel@vger.kernel.org Subject: Re: [PATCH] x86/kvm: Update the comment about asynchronous page fault in exc_page_fault() In-Reply-To: <20201002154313.1505327-1-vkuznets@redhat.com> References: <20201002154313.1505327-1-vkuznets@redhat.com> Date: Mon, 05 Oct 2020 10:42:36 +0200 Message-ID: <87o8lh83nn.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Vitaly Kuznetsov writes: > KVM was switched to interrupt-based mechanism for 'page ready' event > delivery in Linux-5.8 (see commit 2635b5c4a0e4 ("KVM: x86: interrupt based > APF 'page ready' event delivery")) and #PF (ab)use for 'page ready' event > delivery was removed. Linux guest switched to this new mechanism > exclusively in 5.9 (see commit b1d405751cd5 ("KVM: x86: Switch KVM guest to > using interrupts for page ready APF delivery")) so it is not possible to > get older KVM (APF mechanism won't be enabled). Update the comment in Sigh, this should have been "to get #PF for a 'page ready' event even when the guest is running on top of an older KVM" or something like that but the part after "#" was eaten by git :-( > exc_page_fault() to reflect the new reality. > > Signed-off-by: Vitaly Kuznetsov > --- > arch/x86/mm/fault.c | 13 ++++++++----- > 1 file changed, 8 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > index 6e3e8a124903..3cf77592ac54 100644 > --- a/arch/x86/mm/fault.c > +++ b/arch/x86/mm/fault.c > @@ -1446,11 +1446,14 @@ DEFINE_IDTENTRY_RAW_ERRORCODE(exc_page_fault) > prefetchw(¤t->mm->mmap_lock); > > /* > - * KVM has two types of events that are, logically, interrupts, but > - * are unfortunately delivered using the #PF vector. These events are > - * "you just accessed valid memory, but the host doesn't have it right > - * now, so I'll put you to sleep if you continue" and "that memory > - * you tried to access earlier is available now." > + * KVM uses #PF vector to deliver 'page not present' events to guests > + * (asynchronous page fault mechanism). The event happens when a > + * userspace task is trying to access some valid (from guest's point of > + * view) memory which is not currently mapped by the host (e.g. the > + * memory is swapped out). Note, the corresponding "page ready" event > + * which is injected when the memory becomes available, is delived via > + * an interrupt mechanism and not a #PF exception > + * (see arch/x86/kernel/kvm.c: sysvec_kvm_asyncpf_interrupt()). > * > * We are relying on the interrupted context being sane (valid RSP, > * relevant locks not held, etc.), which is fine as long as the -- Vitaly