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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id y24sm3662535ejc.80.2021.09.06.03.32.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 06 Sep 2021 03:33:07 -0700 (PDT) Subject: Re: [PATCH] x86/kvm: Don't enable IRQ when IRQ enabled in kvm_wait To: Lai Jiangshan , linux-kernel@vger.kernel.org Cc: Lai Jiangshan , Mark Rutland , Peter Zijlstra , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , kvm@vger.kernel.org References: <20210814035129.154242-1-jiangshanlai@gmail.com> From: Paolo Bonzini Message-ID: <0d385f5d-c01f-2193-ee0f-54249ee149e7@redhat.com> Date: Mon, 6 Sep 2021 12:32:45 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210814035129.154242-1-jiangshanlai@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 14/08/21 05:51, Lai Jiangshan wrote: > From: Lai Jiangshan > > Commit f4e61f0c9add3 ("x86/kvm: Fix broken irq restoration in kvm_wait") > replaced "local_irq_restore() when IRQ enabled" with "local_irq_enable() > when IRQ enabled" to suppress a warnning. > > Although there is no similar debugging warnning for doing local_irq_enable() > when IRQ enabled as doing local_irq_restore() in the same IRQ situation. But > doing local_irq_enable() when IRQ enabled is no less broken as doing > local_irq_restore() and we'd better avoid it. > > Cc: Mark Rutland > Cc: Peter Zijlstra (Intel) > Signed-off-by: Lai Jiangshan > --- > > The original debugging warnning was introduced in commit 997acaf6b4b5 > ("lockdep: report broken irq restoration"). I think a similar debugging > check and warnning should also be added to "local_irq_enable() when IRQ > enabled" and even maybe "local_irq_disable() when IRQ disabled" to detect > something this: > > | local_irq_save(flags); > | local_irq_disable(); > | local_irq_restore(flags); > | local_irq_enable(); > > Or even we can do the check in lockdep+TRACE_IRQFLAGS: > > In lockdep_hardirqs_on_prepare(), lockdep_hardirqs_enabled() was checked > (and exit) before checking DEBUG_LOCKS_WARN_ON(!irqs_disabled()), so lockdep > can't give any warning for these kind of situations. If we did the check > in lockdep, we would have found the problem before, and we don't need > 997acaf6b4b5. > > Any thought? Mark? Peter? > > arch/x86/kernel/kvm.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c > index a26643dc6bd6..b656456c3a94 100644 > --- a/arch/x86/kernel/kvm.c > +++ b/arch/x86/kernel/kvm.c > @@ -884,10 +884,11 @@ static void kvm_wait(u8 *ptr, u8 val) > } else { > local_irq_disable(); > > + /* safe_halt() will enable IRQ */ > if (READ_ONCE(*ptr) == val) > safe_halt(); > - > - local_irq_enable(); > + else > + local_irq_enable(); > } > } > > Queued, thanks. paolo