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b=QgaxA56OS9ZVriGbwDaZnuW7SkkSadBqDjUO+AD7t4Xv1XW750S1z9k1AYCOBunBuJ3Fae8d7n/V66q5b3KkmccQzMWv6na2MmE5zV2oU2fdmlrGRQCCrQT1iHTBSHQJh2k3N+ptX8xnCgc8Qib6WnT7CAXy+Q8BPjLlzWRWweA= Authentication-Results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=citrix.com; Message-ID: <33aab2bd-db13-43f6-9616-5e75fd79b81d@citrix.com> Date: Mon, 17 Aug 2026 16:55:48 +0100 User-Agent: Mozilla Thunderbird Cc: Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Teddy Astie , Xen-devel Subject: Re: [PATCH] x86/nmi: Fix mis-classification of watchdog NMIs To: Jan Beulich References: <20260813174319.1682009-1-andrew.cooper3@citrix.com> <4f58e236-c40b-4fcc-82e8-5b6f3eafe656@suse.com> 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: <4f58e236-c40b-4fcc-82e8-5b6f3eafe656@suse.com> Content-Type: text/plain; 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Following the rework, cpuid_level has a >> placeholder 1 until it is caluclated propely. >> >> setup_apic_nmi_watchdog() happens to be called on the BSP after SMP bringup, >> meaning that the first call is on CPU1. It is also positioned in the window >> where cpu_data[] is garbage. >> >> As a result, setup_p6_watchdog()'s one-time calculation of the performance >> counter width falls back into Pentium compatibility mode assuming 32bit >> counters. This causes a watchdog NMI which is delayed a little (e.g. from an >> SMI), to appear as if it hadn't overflowed, and therefore be (mis)classifed as >> not a watchdog NMI. On systems where unknown NMIs are treated as fatal, this >> results in a spurious crash. >> >> Switch setup_p6_watchdog() to use boot_cpu_data.cpuid_level, which is how this >> is checked almost everywhere else. >> >> core2_vpmu_init() used the same pattern to look at leaf 0xa. Despite being >> init code and only running on the BSP, {boot,current}_cpu_data are different >> objects, so switch it over to checking boot_cpu_data.cpuid_level too. >> >> Fixes: 7126b7f806d5 ("x86/CPU: re-work populating of cpu_data[]") >> Signed-off-by: Andrew Cooper > Reviewed-by: Jan Beulich Thanks. > >> I hate this fix, but it's the only thing which I consider remotely safe to >> backport. Recent attempts to alter CPUID ordering have 0 success at being >> bug-free. > How that? Collecting CPUID output should be doable almost first thing. There > are no (or in case of doubt: there should not be any) dependencies on about > anything else. Of course re-collecting may still be necessary after ucode > loading. Yet from what you say I must be missing something crucial. I was referring to commits in Xen rearranging the boot sequence with respect to feature handling.  There have been many failures recently. > With that in mind, I'm also questioning the Fixes: tag (without this being a > request to drop or change it): Using another CPU's data isn't much better > than using partially unset data. Unless we assumed full symmetry, at which > point re-obtaining of most data on the APs would be entirely useless. Hence > the issue was pre-existing, with one bug there hiding the issue addressed > here. Copying the BSP is less bad than the current behaviour.  It's not necessarily ideal, but it's a damsight better default than the arbitrary 1 that this patch puts in place. Even with the very old 64bit systems where mixing steppings was commonplace, I'm not aware of a vendor supported combination where max_leaf was different. The issue was not prexisting.  Prior to your rearrangement, the NMI watchdog setup found the counter width in CPUID and used it, without falling back into original Pentium compatibility mode. ~Andrew