From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CC48A171C3 for ; Wed, 1 Nov 2023 17:36:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="1G73i3bp" Received: from mail-yb1-xb4a.google.com (mail-yb1-xb4a.google.com [IPv6:2607:f8b0:4864:20::b4a]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 6AAD8C1 for ; Wed, 1 Nov 2023 10:36:50 -0700 (PDT) Received: by mail-yb1-xb4a.google.com with SMTP id 3f1490d57ef6-da0cb98f66cso6455443276.2 for ; Wed, 01 Nov 2023 10:36:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1698860209; x=1699465009; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=Aa5pPv4/g+I3C3eNDgudVhrFstr6S8Xmtwrf0OlljDI=; b=1G73i3bpqP3lEIiYf1Xa+PqF+ry3FPPmHtzhkTz2ESLqKv/PJo83UzY0L3tGLSLtgw FEf6apQxu3ajl2eaf2jpMXM8oiMYlwvL+Gaj7CmFke3L5WVAzOh6QUSsRjnNhhvyggQP /0obTqP8SXrHs5tm5Lcs1IoffSqT4ZAyvwPXIthiiQ2/DYs3pRsDS0oN02OgNcKyJmxV +il1EDZ1G9lZ1tby+sgxcieBPfodwSlNZ7rJ6OOgSa83N+qrWIIICrK86C1Ay71kdg1m h7J1Ii0mSVbVxFFpS5Ul/5maHoLcclIuFP3aDRfEoQUS+LZ7DrcFMxeDrV3szI4QTAZm WoHw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1698860209; x=1699465009; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=Aa5pPv4/g+I3C3eNDgudVhrFstr6S8Xmtwrf0OlljDI=; b=lfOpJD2eZJbQMUL4u3/duTStw/DZKZ3qnWXm9HbrmXR2Sh0QR3DFpdicRvVz/VF72B 8KWztgWYa7V8UrhfkotaveZFlAuSQcfstzaVEqntujayT12LfwBIhmKTaecGZD9uMgwL VFlJUMc0K8MCtbKRyNGLZ+6vw2XQkj3/Qh/9XqQt4hv59j3oOX5wLSKjQhRp2wh7LlCq 4HS7yZc6egaql47WCVTWwtLk0FBjhY76oI8OaHbjrtNzooRe5FYv0TJKTmLE5Di2vJji U9K58fwtmaCNxyqJjEsOVyJ6RNLGqDnj6gVNqc/LpgGGNEA9jJ60YgCIU3ySLH2qwrDc zRPQ== X-Gm-Message-State: AOJu0YzlAkvnbrmB3QaByJ/N2K8vEgcqf41nuDVGORRgdDWtTzr4MsV+ rdGTM72zon++8WwffGiuFQnoSrbZQvY= X-Google-Smtp-Source: AGHT+IG/a6Sj0mASs5/8PEJQnR69sSphX+9SHKWYEjRPOLGEF4l6c52AgzjYtmdhHr7JL68mzkGP7JI8NEg= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a05:6902:168c:b0:da0:3e46:8ba5 with SMTP id bx12-20020a056902168c00b00da03e468ba5mr304342ybb.8.1698860209654; Wed, 01 Nov 2023 10:36:49 -0700 (PDT) Date: Wed, 1 Nov 2023 10:36:48 -0700 In-Reply-To: <482bfea6f54ea1bb7d1ad75e03541d0ba0e5be6f.camel@intel.com> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20231027182217.3615211-1-seanjc@google.com> <20231027182217.3615211-10-seanjc@google.com> <482bfea6f54ea1bb7d1ad75e03541d0ba0e5be6f.camel@intel.com> Message-ID: Subject: Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace From: Sean Christopherson To: Kai Huang Cc: "viro@zeniv.linux.org.uk" , "aou@eecs.berkeley.edu" , "brauner@kernel.org" , "oliver.upton@linux.dev" , "chenhuacai@kernel.org" , "paul.walmsley@sifive.com" , "palmer@dabbelt.com" , "maz@kernel.org" , "pbonzini@redhat.com" , "mpe@ellerman.id.au" , "willy@infradead.org" , "anup@brainfault.org" , "akpm@linux-foundation.org" , Xiaoyao Li , "kvm-riscv@lists.infradead.org" , "mic@digikod.net" , "liam.merwick@oracle.com" , "kvm@vger.kernel.org" , Isaku Yamahata , "kirill.shutemov@linux.intel.com" , "david@redhat.com" , "tabba@google.com" , "amoorthy@google.com" , "linuxppc-dev@lists.ozlabs.org" , "michael.roth@amd.com" , "kvmarm@lists.linux.dev" , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-riscv@lists.infradead.org" , "chao.p.peng@linux.intel.com" , "linux-mips@vger.kernel.org" , Vishal Annapurve , "vbabka@suse.cz" , "mail@maciej.szmigiero.name" , "yu.c.zhang@linux.intel.com" , "qperret@google.com" , "dmatlack@google.com" , Yilun Xu , "isaku.yamahata@gmail.com" , "ackerleytng@google.com" , "jarkko@kernel.org" , "linux-arm-kernel@lists.infradead.org" , "linux-mm@kvack.org" , Wei W Wang Content-Type: text/plain; charset="us-ascii" On Wed, Nov 01, 2023, Kai Huang wrote: > > > +7.34 KVM_CAP_MEMORY_FAULT_INFO > > +------------------------------ > > + > > +:Architectures: x86 > > +:Returns: Informational only, -EINVAL on direct KVM_ENABLE_CAP. > > + > > +The presence of this capability indicates that KVM_RUN will fill > > +kvm_run.memory_fault if KVM cannot resolve a guest page fault VM-Exit, e.g. if > > +there is a valid memslot but no backing VMA for the corresponding host virtual > > +address. > > + > > +The information in kvm_run.memory_fault is valid if and only if KVM_RUN returns > > +an error with errno=EFAULT or errno=EHWPOISON *and* kvm_run.exit_reason is set > > +to KVM_EXIT_MEMORY_FAULT. > > IIUC returning -EFAULT or whatever -errno is sort of KVM internal > implementation. The errno that is returned to userspace is ABI. In KVM, it's a _very_ poorly defined ABI for the vast majority of ioctls(), but it's still technically ABI. KVM gets away with being cavalier with errno because the vast majority of errors are considered fatal by userespace, i.e. in most cases, userspace simply doesn't care about the exact errno. A good example is KVM_RUN with -EINTR; if KVM were to return something other than -EINTR on a pending signal or vcpu->run->immediate_exit, userspace would fall over. > Is it better to relax the validity of kvm_run.memory_fault when > KVM_RUN returns any -errno? Not unless there's a need to do so, and if there is then we can update the documentation accordingly. If KVM's ABI is that kvm_run.memory_fault is valid for any errno, then KVM would need to purge kvm_run.exit_reason super early in KVM_RUN, e.g. to prevent an -EINTR return due to immediate_exit from being misinterpreted as KVM_EXIT_MEMORY_FAULT. And purging exit_reason super early is subtly tricky because KVM's (again, poorly documented) ABI is that *some* exit reasons are preserved across KVM_RUN with vcpu->run->immediate_exit (or with a pending signal). https://lore.kernel.org/all/ZFFbwOXZ5uI%2Fgdaf@google.com > [...] > > > > --- a/include/linux/kvm_host.h > > +++ b/include/linux/kvm_host.h > > @@ -2327,4 +2327,15 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr) > > /* Max number of entries allowed for each kvm dirty ring */ > > #define KVM_DIRTY_RING_MAX_ENTRIES 65536 > > > > +static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu, > > + gpa_t gpa, gpa_t size) > > +{ > > + vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT; > > + vcpu->run->memory_fault.gpa = gpa; > > + vcpu->run->memory_fault.size = size; > > + > > + /* Flags are not (yet) defined or communicated to userspace. */ > > + vcpu->run->memory_fault.flags = 0; > > +} > > + > > KVM_CAP_MEMORY_FAULT_INFO is x86 only, is it better to put this function to > ? I'd prefer to keep it in generic code, as it's highly likely to end up there sooner than later. There's a known use case for ARM (exit to userspace on missing userspace mapping[*]), and I'm guessing pKVM (also ARM) will also utilize this API. [*] https://lore.kernel.org/all/20230908222905.1321305-8-amoorthy@google.com