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b=Mpti6Mnw7jNgvGzZFYzld/fUxov/yPFGSMth5BHUFes8PhqndGWfR8XIS1vYJ5vzv9ZlVLVK6DAZHkxhYjGN/OXILsR0OTjRORwhcM/Rre+HViorDBXJ/bqvP9bPm1quh7BSmG/jE7zX1s2u/6YnSTGb1GubpX7bm3o+9UbSdUI= Authentication-Results: mx.microsoft.com 1; dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=citrix.com; Message-ID: Date: Fri, 23 Oct 2026 21:45:27 +0100 User-Agent: Mozilla Thunderbird Cc: Andrew Cooper , Jan Beulich , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Subject: Re: [PATCH] x86/vlapic: Don't change ICR on x2APIC SELF_IPI write To: Teddy Astie , xen-devel@lists.xenproject.org References: <1792420048.8631fc262581453bbf619ec5b2062170.1a1548f6f6b0008425@vates.tech> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; keydata= xsFNBFLhNn8BEADVhE+Hb8i0GV6mihnnr/uiQQdPF8kUoFzCOPXkf7jQ5sLYeJa0cQi6Penp VtiFYznTairnVsN5J+ujSTIb+OlMSJUWV4opS7WVNnxHbFTPYZVQ3erv7NKc2iVizCRZ2Kxn srM1oPXWRic8BIAdYOKOloF2300SL/bIpeD+x7h3w9B/qez7nOin5NzkxgFoaUeIal12pXSR Q354FKFoy6Vh96gc4VRqte3jw8mPuJQpfws+Pb+swvSf/i1q1+1I4jsRQQh2m6OTADHIqg2E ofTYAEh7R5HfPx0EXoEDMdRjOeKn8+vvkAwhviWXTHlG3R1QkbE5M/oywnZ83udJmi+lxjJ5 YhQ5IzomvJ16H0Bq+TLyVLO/VRksp1VR9HxCzItLNCS8PdpYYz5TC204ViycobYU65WMpzWe LFAGn8jSS25XIpqv0Y9k87dLbctKKA14Ifw2kq5OIVu2FuX+3i446JOa2vpCI9GcjCzi3oHV e00bzYiHMIl0FICrNJU0Kjho8pdo0m2uxkn6SYEpogAy9pnatUlO+erL4LqFUO7GXSdBRbw5 gNt25XTLdSFuZtMxkY3tq8MFss5QnjhehCVPEpE6y9ZjI4XB8ad1G4oBHVGK5LMsvg22PfMJ ISWFSHoF/B5+lHkCKWkFxZ0gZn33ju5n6/FOdEx4B8cMJt+cWwARAQABzSlBbmRyZXcgQ29v cGVyIDxhbmRyZXcuY29vcGVyM0BjaXRyaXguY29tPsLBegQTAQgAJAIbAwULCQgHAwUVCgkI CwUWAgMBAAIeAQIXgAUCWKD95wIZAQAKCRBlw/kGpdefoHbdD/9AIoR3k6fKl+RFiFpyAhvO 59ttDFI7nIAnlYngev2XUR3acFElJATHSDO0ju+hqWqAb8kVijXLops0gOfqt3VPZq9cuHlh IMDquatGLzAadfFx2eQYIYT+FYuMoPZy/aTUazmJIDVxP7L383grjIkn+7tAv+qeDfE+txL4 SAm1UHNvmdfgL2/lcmL3xRh7sub3nJilM93RWX1Pe5LBSDXO45uzCGEdst6uSlzYR/MEr+5Z JQQ32JV64zwvf/aKaagSQSQMYNX9JFgfZ3TKWC1KJQbX5ssoX/5hNLqxMcZV3TN7kU8I3kjK mPec9+1nECOjjJSO/h4P0sBZyIUGfguwzhEeGf4sMCuSEM4xjCnwiBwftR17sr0spYcOpqET ZGcAmyYcNjy6CYadNCnfR40vhhWuCfNCBzWnUW0lFoo12wb0YnzoOLjvfD6OL3JjIUJNOmJy RCsJ5IA/Iz33RhSVRmROu+TztwuThClw63g7+hoyewv7BemKyuU6FTVhjjW+XUWmS/FzknSi dAG+insr0746cTPpSkGl3KAXeWDGJzve7/SBBfyznWCMGaf8E2P1oOdIZRxHgWj0zNr1+ooF /PzgLPiCI4OMUttTlEKChgbUTQ+5o0P080JojqfXwbPAyumbaYcQNiH1/xYbJdOFSiBv9rpt TQTBLzDKXok86M7BTQRS4TZ/ARAAkgqudHsp+hd82UVkvgnlqZjzz2vyrYfz7bkPtXaGb9H4 Rfo7mQsEQavEBdWWjbga6eMnDqtu+FC+qeTGYebToxEyp2lKDSoAsvt8w82tIlP/EbmRbDVn 7bhjBlfRcFjVYw8uVDPptT0TV47vpoCVkTwcyb6OltJrvg/QzV9f07DJswuda1JH3/qvYu0p vjPnYvCq4NsqY2XSdAJ02HrdYPFtNyPEntu1n1KK+gJrstjtw7KsZ4ygXYrsm/oCBiVW/OgU g/XIlGErkrxe4vQvJyVwg6YH653YTX5hLLUEL1NS4TCo47RP+wi6y+TnuAL36UtK/uFyEuPy wwrDVcC4cIFhYSfsO0BumEI65yu7a8aHbGfq2lW251UcoU48Z27ZUUZd2Dr6O/n8poQHbaTd 6bJJSjzGGHZVbRP9UQ3lkmkmc0+XCHmj5WhwNNYjgbbmML7y0fsJT5RgvefAIFfHBg7fTY/i kBEimoUsTEQz+N4hbKwo1hULfVxDJStE4sbPhjbsPCrlXf6W9CxSyQ0qmZ2bXsLQYRj2xqd1 bpA+1o1j2N4/au1R/uSiUFjewJdT/LX1EklKDcQwpk06Af/N7VZtSfEJeRV04unbsKVXWZAk uAJyDDKN99ziC0Wz5kcPyVD1HNf8bgaqGDzrv3TfYjwqayRFcMf7xJaL9xXedMcAEQEAAcLB XwQYAQgACQUCUuE2fwIbDAAKCRBlw/kGpdefoG4XEACD1Qf/er8EA7g23HMxYWd3FXHThrVQ HgiGdk5Yh632vjOm9L4sd/GCEACVQKjsu98e8o3ysitFlznEns5EAAXEbITrgKWXDDUWGYxd pnjj2u+GkVdsOAGk0kxczX6s+VRBhpbBI2PWnOsRJgU2n10PZ3mZD4Xu9kU2IXYmuW+e5KCA vTArRUdCrAtIa1k01sPipPPw6dfxx2e5asy21YOytzxuWFfJTGnVxZZSCyLUO83sh6OZhJkk b9rxL9wPmpN/t2IPaEKoAc0FTQZS36wAMOXkBh24PQ9gaLJvfPKpNzGD8XWR5HHF0NLIJhgg 4ZlEXQ2fVp3XrtocHqhu4UZR4koCijgB8sB7Tb0GCpwK+C4UePdFLfhKyRdSXuvY3AHJd4CP 4JzW0Bzq/WXY3XMOzUTYApGQpnUpdOmuQSfpV9MQO+/jo7r6yPbxT7CwRS5dcQPzUiuHLK9i nvjREdh84qycnx0/6dDroYhp0DFv4udxuAvt1h4wGwTPRQZerSm4xaYegEFusyhbZrI0U9tJ B8WrhBLXDiYlyJT6zOV2yZFuW47VrLsjYnHwn27hmxTC/7tvG3euCklmkn9Sl9IAKFu29RSo d5bD8kMSCYsTqtTfT6W4A3qHGvIDta3ptLYpIAOD2sY3GYq2nf3Bbzx81wZK14JdDDHUX2Rs 6+ahAA== In-Reply-To: <1792420048.8631fc262581453bbf619ec5b2062170.1a1548f6f6b0008425@vates.tech> Content-Type: text/plain; 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Having spoken to Intel, all Core/P-core's implement SELF_IPI using ICR, so do mutate the value. More recent Atom/E-core designs implement SELF_IPI differently, and do not mutate ICR.  From the testing I performed in Gitlab-CI, "More recent" is at least AlderLake and later, so Gracemont cores. Either way, this is conformation that both behaviours are legitimate, and it is certainly not a "bugfix" to change the behaviour. There might be enough reason to change the behaviour, as the Atom behaviour "feels" cleaner, and because it reduces the work we need to do in the SELF_IPI path, but a justification needs to be along these lines, not in terms of being a bugfix. Perhaps a better reason to make the change is because the VT-x architecture appears to not overwrite ICR, even on Core designs, and this would be a behavioural difference between APICV or not. > Fix the Xen behavior by sending the Self-IPI directly instead of > emulating it by modifying ICR register. > > Fixes: 21c4d089ba2a ("x86 hvm: x2APIC emulation") > Assisted-by: Claude Fable 5.1 > Signed-off-by: Teddy Astie > --- > Linux/KVM has a similar patch : > > KVM: x86: Do not change ICR on write to APIC_SELF_IPI > https://github.com/torvalds/linux/commit/d22a81b304a27fca6124174a8e842e826c193466 > > xen/arch/x86/hvm/vlapic.c | 21 ++++++++++++++++----- > 1 file changed, 16 insertions(+), 5 deletions(-) > > diff --git a/xen/arch/x86/hvm/vlapic.c b/xen/arch/x86/hvm/vlapic.c > index 885f5d304b..bb105225e2 100644 > --- a/xen/arch/x86/hvm/vlapic.c > +++ b/xen/arch/x86/hvm/vlapic.c > @@ -963,6 +963,17 @@ static int cf_check vlapic_mmio_write( > return X86EMUL_OKAY; > } > > +/* > + * A write to the SELF IPI register sends a fixed, edge triggered IPI with > + * the given vector to the local APIC itself. Unlike an ICR write it doesn't > + * update the ICR, which is guest visible, in x2APIC mode even without any > + * intervention by Xen when APIC-register virtualization is in use. > + */ > +static void vlapic_self_ipi(struct vlapic *vlapic, uint32_t val) > +{ > + vlapic_ipi(vlapic, APIC_DEST_SELF | (val & APIC_VECTOR_MASK), 0); > +} I'm not sure a wrapper is really worth it, but if you do care to continue with the patch, then you want a uint8_t vec here, and drop the mask. ~Andrew