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b=VfQFmH1Oo9huXczCLKANlbLIrNrMYzLgsHTEYjeEyx5VYr8ZItOD39fo49h+pRKRGJGu3o6b9AB354Q0k1GP0j1SDQuJ7gNghCTGHhIc6CCu7nUk7ShiVN+WDYIkNB1ltBw0FhQzGg0QDZ/UIVkY1v/c42TKT+iLzsPB7aNYCGE= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=citrix.com; Received: from CH8PR03MB8275.namprd03.prod.outlook.com (2603:10b6:610:2b9::7) by LV4PR03MB8236.namprd03.prod.outlook.com (2603:10b6:408:2e0::17) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9611.10; Wed, 11 Feb 2026 13:55:39 +0000 Received: from CH8PR03MB8275.namprd03.prod.outlook.com ([fe80::a70d:dc32:bba8:ce37]) by CH8PR03MB8275.namprd03.prod.outlook.com ([fe80::a70d:dc32:bba8:ce37%6]) with mapi id 15.20.9611.008; Wed, 11 Feb 2026 13:55:39 +0000 Message-ID: <5276256b-9669-46df-8fcd-b216f3d3e45b@citrix.com> Date: Wed, 11 Feb 2026 13:55:35 +0000 User-Agent: Mozilla Thunderbird Cc: Andrew Cooper , ubizjak@gmail.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@kernel.org, pbonzini@redhat.com, seanjc@google.com, tglx@kernel.org, x86@kernel.org Subject: Re: [PATCH 1/2] KVM: VMX: Drop obsolete branch hint prefixes from inline asm To: David Laight References: <20260211102928.100944-1-ubizjak@gmail.com> <2af5e3a8-f520-40fd-96a5-28555c3e4a5e@citrix.com> <20260211134342.45b7e19e@pumpkin> Content-Language: en-GB From: Andrew Cooper In-Reply-To: <20260211134342.45b7e19e@pumpkin> Content-Type: text/plain; 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Keeping them provides no measurable benefit and only >>> enlarges the generated code. >> It's actually worse than this. >> >> The branch-taken hint has new meaning in Lion Cove cores and later, >> along with a warning saying "performance penalty for misuse". >> >> i.e. "only insert this prefix after profiling". > Don't they really have much the same meaning as before? Architecturally yes, microarchitecturally very much not. For a branch known to the predictor, there is no effect.  If a branch unknown to the predictor gets decoded, it triggers a frontend flush and resteer. It is only useful for programs large enough to exceed the working set of the conditional predictor, and for which certain branches are known to be ~always taken. Putting the prefix on a branch that isn't ~always taken is worse than not having the prefix in the first place, hence the warning. ~Andrew