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Fri, 13 Dec 2024 10:58:55 -0800 (PST) X-Google-Smtp-Source: AGHT+IGCocN09lwrJ18yznQg6RRv3t1XmTAkjMEIYeGuE67QjSo4SyJA5WAXCajyTNxHaPEkuz0Z7A== X-Received: by 2002:a05:600c:190c:b0:42c:c28c:e477 with SMTP id 5b1f17b1804b1-4362aa9d675mr34182965e9.23.1734116335622; Fri, 13 Dec 2024 10:58:55 -0800 (PST) Received: from [192.168.10.3] ([151.81.118.45]) by smtp.googlemail.com with ESMTPSA id ffacd0b85a97d-388c80602a1sm251014f8f.97.2024.12.13.10.58.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 13 Dec 2024 10:58:55 -0800 (PST) Message-ID: <257491b6-f70a-4347-b97d-cc7fa22aac85@redhat.com> Date: Fri, 13 Dec 2024 19:58:54 +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 0/5] KVM: x86: Address xstate_required_size() perf regression To: Sean Christopherson Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Jim Mattson References: <20241211013302.1347853-1-seanjc@google.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 12/11/24 02:32, Sean Christopherson wrote: > Fix a hilarious/revolting performance regression (relative to older CPU > generations) in xstate_required_size() that pops up due to CPUID _in the > host_ taking 3x-4x longer on Emerald Rapids than Skylake. > > The issue rears its head on nested virtualization transitions, as KVM > (unnecessarily) performs runtime CPUID updates, including XSAVE sizes, > multiple times per transition. And calculating XSAVE sizes, especially > for vCPUs with a decent number of supported XSAVE features and compacted > format support, can add up to thousands of cycles. > > To fix the immediate issue, cache the CPUID output at kvm.ko load. The > information is static for a given CPU, i.e. doesn't need to be re-read > from hardware every time. That's patch 1, and eliminates pretty much all > of the meaningful overhead. Queued this one, thanks! Paolo