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[70.27.3.10]) by smtp.gmail.com with ESMTPSA id z6-20020a05663822a600b0034294118e1bsm215302jas.126.2022.08.11.13.58.07 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 11 Aug 2022 13:58:08 -0700 (PDT) Date: Thu, 11 Aug 2022 16:58:07 -0400 From: Peter Xu To: Sean Christopherson Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, David Hildenbrand , "Dr . David Alan Gilbert" , John Hubbard , Linux MM Mailing List , Andrew Morton , Paolo Bonzini , Andrea Arcangeli Subject: Re: [PATCH v2 3/3] kvm/x86: Allow to respond to generic signals during slow page faults Message-ID: References: <20220721000318.93522-1-peterx@redhat.com> <20220721000318.93522-4-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 11, 2022 at 08:12:38PM +0000, Sean Christopherson wrote: > On Wed, Jul 20, 2022, Peter Xu wrote: > > All the facilities should be ready for this, what we need to do is to add a > > new "interruptible" flag showing that we're willing to be interrupted by > > common signals during the __gfn_to_pfn_memslot() request, and wire it up > > with a FOLL_INTERRUPTIBLE flag that we've just introduced. > > > > Note that only x86 slow page fault routine will set this to true. The new > > flag is by default false in non-x86 arch or on other gup paths even for > > x86. It can actually be used elsewhere too but not yet covered. > > > > When we see the PFN fetching was interrupted, do early exit to userspace > > with an KVM_EXIT_INTR exit reason. > > > > Signed-off-by: Peter Xu > > --- > > arch/arm64/kvm/mmu.c | 2 +- > > arch/powerpc/kvm/book3s_64_mmu_hv.c | 2 +- > > arch/powerpc/kvm/book3s_64_mmu_radix.c | 2 +- > > arch/x86/kvm/mmu/mmu.c | 16 ++++++++++++-- > > include/linux/kvm_host.h | 4 ++-- > > virt/kvm/kvm_main.c | 30 ++++++++++++++++---------- > > virt/kvm/kvm_mm.h | 4 ++-- > > virt/kvm/pfncache.c | 2 +- > > 8 files changed, 41 insertions(+), 21 deletions(-) > > I don't usually like adding code without a user, but in this case I think I'd > prefer to add the @interruptible param and then activate x86's kvm_faultin_pfn() > in a separate patch. It's rather difficult to tease out the functional x86 > change, and that would also allow other architectures to use the interruptible > support without needing to depend on the functional x86 change. > > And maybe squash the addition of @interruptible with the previous patch? I.e. > add all of the infrastructure for KVM_PFN_ERR_SIGPENDING in patch 2, then use it > in x86 in patch 3. Sounds good. > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > > index 17252f39bd7c..aeafe0e9cfbf 100644 > > --- a/arch/x86/kvm/mmu/mmu.c > > +++ b/arch/x86/kvm/mmu/mmu.c > > @@ -3012,6 +3012,13 @@ static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn) > > static int handle_abnormal_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault, > > unsigned int access) > > { > > + /* NOTE: not all error pfn is fatal; handle sigpending pfn first */ > > + if (unlikely(is_sigpending_pfn(fault->pfn))) { > > Move this into kvm_handle_bad_page(), then there's no need for a comment to call > out that this needs to come before the is_error_pfn() check. This _is_ a "bad" > PFN, it just so happens that userspace might be able to resolve the "bad" PFN. It's a pity it needs to be in "bad pfn" category since that's the only thing we can easily use, but true it is now. > > > + vcpu->run->exit_reason = KVM_EXIT_INTR; > > + ++vcpu->stat.signal_exits; > > + return -EINTR; > > For better or worse, kvm_handle_signal_exit() exists and can be used here. I > don't love that KVM details bleed into xfer_to_guest_mode_work(), but that's a > future problem. > > I do think that the "return -EINTR" should be moved into kvm_handle_signal_exit(), > partly for code reuse and partly because returning -EINTR is very much KVM ABI. > Oof, but there are a _lot_ of paths that can use kvm_handle_signal_exit(), and > some of them don't select KVM_XFER_TO_GUEST_WORK, i.e. kvm_handle_signal_exit() > should be defined unconditionally. I'll work on a series to handle that separately, > no reason to take a dependency on that cleanup. > > So for now, > > static int kvm_handle_bad_page(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t pfn) > { > if (pfn == KVM_PFN_ERR_SIGPENDING) { > kvm_handle_signal_exit(vcpu); > return -EINTR; > } > > ... > } Sounds good too here. Also all points taken in the wording of patch 2. Will respin shortly, thanks Sean. -- Peter Xu