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Wed, 02 Sep 2026 05:45:53 -0700 (PDT) From: Paul Durrant X-Google-Original-From: Paul Durrant Message-ID: Date: Wed, 2 Sep 2026 13:45:52 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 11/13] KVM: x86/xen: Mark poll_evtchn accesses with READ_ONCE()/WRITE_ONCE() To: David Woodhouse , seanjc@google.com, pbonzini@redhat.com Cc: joao.m.martins@oracle.com, boris.ostrovsky@oracle.com, ankur.a.arora@oracle.com, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, x86@kernel.org, syzbot+208f7f3e5f59c11aeb90@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com, suryasaimadhu369@gmail.com, lkp@intel.com, nicoyip.dev@gmail.com, frn1furkan10@gmail.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, imv4bel@gmail.com References: <20260831213632.81023-1-dwmw2@infradead.org> <20260831213632.81023-12-dwmw2@infradead.org> Content-Language: en-US In-Reply-To: <20260831213632.81023-12-dwmw2@infradead.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 31/08/2026 22:26, David Woodhouse wrote: > From: Chengfeng Ye > > Use READ_ONCE() and WRITE_ONCE() for runtime accesses to poll_evtchn. > This marks the intentionally concurrent scalar accesses and prevents the > compiler from splitting, merging, or inventing accesses. > > kvm_xen_schedop_poll() publishes the single port, or -1 for multiple > ports, before setting poll_mask and halting the vCPU. Event delivery can > call kvm_xen_check_poller() on another CPU while the vCPU thread publishes > that value or resets the field to zero after returning from > kvm_vcpu_halt(): > > vCPU thread event delivery thread > ----------- --------------------- > poll_evtchn = port > set_bit(poll_mask) > kvm_vcpu_halt() > poll_evtchn = READ > poll_evtchn = 0 > clear_bit(poll_mask) > > The plain read and writes therefore race. KCSAN reported: > > BUG: KCSAN: data-race in kvm_xen_hypercall / kvm_xen_set_evtchn_fast > > read to 0xffff888112f55af0 of 4 bytes by task 98: > kvm_xen_set_evtchn_fast+0x204/0x7c0 > kvm_xen_hvm_evtchn_send+0xab/0x100 > kvm_arch_vm_ioctl+0xb31/0xd90 > kvm_vm_ioctl+0xf42/0x16c0 > > write to 0xffff888112f55af0 of 4 bytes by task 96: > kvm_xen_hypercall+0xd8d/0xf50 > kvm_emulate_hypercall+0x157/0x1d0 > vmx_handle_exit+0x40f/0xae0 > vcpu_run+0x137f/0x27d0 > kvm_arch_vcpu_ioctl_run+0x5a5/0x970 > > The field is an aligned int on x86. Access annotations preserve the > existing matching, callback, and poll-mask control flow while making the > single-copy access requirement explicit. > > Fixes: 1a65105a5aba ("KVM: x86/xen: handle PV spinlocks slowpath") > Signed-off-by: Chengfeng Ye > Signed-off-by: David Woodhouse > --- > arch/x86/kvm/xen.c | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > Reviewed-by: Paul Durrant