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Wed, 15 May 2024 11:59:43 -0700 (PDT) Message-ID: Date: Wed, 15 May 2024 19:59:41 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH for-4.19] x86/mtrr: avoid system wide rendezvous when setting AP MTRRs To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: xen-devel@lists.xenproject.org, Jan Beulich , Oleksii Kurochko References: <20240513085925.59324-1-roger.pau@citrix.com> <2d245c04-3bf9-4b9d-ad02-e754dcbefa28@citrix.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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 14/05/2024 3:43 pm, Roger Pau Monné wrote: > On Tue, May 14, 2024 at 02:50:18PM +0100, Andrew Cooper wrote: >> On 14/05/2024 12:09 pm, Andrew Cooper wrote: >>> On 13/05/2024 9:59 am, Roger Pau Monne wrote: >>>> There's no point in forcing a system wide update of the MTRRs on all processors >>>> when there are no changes to be propagated. On AP startup it's only the AP >>>> that needs to write the system wide MTRR values in order to match the rest of >>>> the already online CPUs. >>>> >>>> We have occasionally seen the watchdog trigger during `xen-hptool cpu-online` >>>> in one Intel Cascade Lake box with 448 CPUs due to the re-setting of the MTRRs >>>> on all the CPUs in the system. >>>> >>>> While there adjust the comment to clarify why the system-wide resetting of the >>>> MTRR registers is not needed for the purposes of mtrr_ap_init(). >>>> >>>> Signed-off-by: Roger Pau Monné >>>> --- >>>> For consideration for 4.19: it's a bugfix of a rare instance of the watchdog >>>> triggering, but it's also a good performance improvement when performing >>>> cpu-online. >>>> >>>> Hopefully runtime changes to MTRR will affect a single MSR at a time, lowering >>>> the chance of the watchdog triggering due to the system-wide resetting of the >>>> range. >>> "Runtime" changes will only be during dom0 boot, if at all, but yes - it >>> is restricted to a single MTRR at a time. >>> >>> It's XENPF_{add,del,read}_memtype, but it's only used by Classic Linux.  >>> PVOps only issues read_memtype. >>> >>> Acked-by: Andrew Cooper >> Actually no - this isn't safe in all cases. >> >> There are BIOSes which get MTRRs wrong, and with the APs having UC >> covering a wider region than the BSP. >> >> In this case, creating consistency will alter the MTRRs on all CPUs >> currently up, and we do need to perform the rendezvous in that case. > I'm confused, the state that gets applied in mtrr_set_all() is not > modified to match what's in the started AP registers. > > An AP starting with a different set of MTRR registers than the saved > state will result in the MTRR state on the AP being changed, but not > the Xen state stored in mtrr_state, and hence there will be no changes > to synchronize. > >> There are 3 cases: >> >> 1) Nothing to do.  This is the overwhemlingly common case. >> 2) Local changes only.  No broadcast, but we do need to enter CD mode. >> 3) Remote changes needed.  Needs full broadcast. > Please bear with me, but I don't think 3) is possible during AP > bringup. It's possible I'm missing a path where the differences in > the started AP MTRR state are somehow reconciled with the cached MTRR > state? [Summarising a conversation on Matrix] The problem case is when the BSP has an MTRR covering [$X, $X+2) and an AP has has an MTRR covering [$X, $X+3). This is a firmware bug (asymmetric settings), but it has been observed in practice.  The resolution in this case ought to be to update all CPUs to use [$X, $X+3), if that is the more UC direction. However, it appears that Xen always resolves asymmetry like this by choosing the BSP setting.  Therefore, (whether we should or not), we don't have a case where observing an AP state results in a change of state on other CPUs. Therefore while case 3 exists in reality, we're not losing it as a side effect of this patch. So we'll take the improvement here and defer the other bugs to a future juncture. ~Andrew