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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id w5sm356911wmg.42.2020.10.19.08.17.58 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 19 Oct 2020 08:17:58 -0700 (PDT) Subject: Re: [PATCH] x86/kvm: Update the comment about asynchronous page fault in exc_page_fault() To: Vitaly Kuznetsov , x86@kernel.org Cc: kvm@vger.kernel.org, Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Peter Zijlstra , linux-kernel@vger.kernel.org References: <20201002154313.1505327-1-vkuznets@redhat.com> From: Paolo Bonzini Message-ID: <8ee54faa-b204-ccec-da42-8029f834b80e@redhat.com> Date: Mon, 19 Oct 2020 17:17:57 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1 MIME-Version: 1.0 In-Reply-To: <20201002154313.1505327-1-vkuznets@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/10/20 17:43, Vitaly Kuznetsov wrote: > 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 > 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 > Queued, thanks. Paolo