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Date: Thu, 14 Jul 2022 07:30:45 +0200 From: Thorsten Leemhuis <regressions@...mhuis.info> To: Chuck Zmudzinski <brchuckz@....com>, linux-kernel@...r.kernel.org Cc: Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Dan Williams <dan.j.williams@...el.com>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Tom Lendacky <thomas.lendacky@....com>, Jane Chu <jane.chu@...cle.com>, Tianyu Lan <Tianyu.Lan@...rosoft.com>, Randy Dunlap <rdunlap@...radead.org>, Sean Christopherson <seanjc@...gle.com>, Jan Beulich <jbeulich@...e.com>, Juergen Gross <jgross@...e.com>, xen-devel@...ts.xenproject.org, stable@...r.kernel.org Subject: Re: [PATCH v2] Subject: x86/PAT: Report PAT on CPUs that support PAT without MTRR On 13.07.22 03:36, Chuck Zmudzinski wrote: > The commit 99c13b8c8896d7bcb92753bf > ("x86/mm/pat: Don't report PAT on CPUs that don't support it") > incorrectly failed to account for the case in init_cache_modes() when > CPUs do support PAT and falsely reported PAT to be disabled when in > fact PAT is enabled. In some environments, notably in Xen PV domains, > MTRR is disabled but PAT is still enabled, and that is the case > that the aforementioned commit failed to account for. > > As an unfortunate consequnce, the pat_enabled() function currently does > not correctly report that PAT is enabled in such environments. The fix > is implemented in init_cache_modes() by setting pat_bp_enabled to true > in init_cache_modes() for the case that commit 99c13b8c8896d7bcb92753bf > ("x86/mm/pat: Don't report PAT on CPUs that don't support it") failed > to account for. > > This approach arranges for pat_enabled() to return true in the Xen PV > environment without undermining the rest of PAT MSR management logic > that considers PAT to be disabled: Specifically, no writes to the PAT > MSR should occur. > > This patch fixes a regression that some users are experiencing with > Linux as a Xen Dom0 driving particular Intel graphics devices by > correctly reporting to the Intel i915 driver that PAT is enabled where > previously it was falsely reporting that PAT is disabled. Some users > are experiencing system hangs in Xen PV Dom0 and all users on Xen PV > Dom0 are experiencing reduced graphics performance because the keying of > the use of WC mappings to pat_enabled() (see arch_can_pci_mmap_wc()) > means that in particular graphics frame buffer accesses are quite a bit > less performant than possible without this patch. > > Also, with the current code, in the Xen PV environment, PAT will not be > disabled if the administrator sets the "nopat" boot option. Introduce > a new boolean variable, pat_force_disable, to forcibly disable PAT > when the administrator sets the "nopat" option to override the default > behavior of using the PAT configuration that Xen has provided. > > For the new boolean to live in .init.data, init_cache_modes() also needs > moving to .init.text (where it could/should have lived already before). > > Fixes: 99c13b8c8896d7bcb92753bf ("x86/mm/pat: Don't report PAT on CPUs that don't support it") BTW, "submitting-patches.rst" says it should just be "the first 12 characters of the SHA-1 ID" > Co-developed-by: Jan Beulich <jbeulich@...e.com> > Cc: stable@...r.kernel.org > Signed-off-by: Chuck Zmudzinski <brchuckz@....com> Sorry, have to ask: is this supposed to finally fix this regression? https://lore.kernel.org/regressions/YnHK1Z3o99eMXsVK@mail-itl/ If yes, please include Link: and Reported-by: tags, as explained in submitting-patches.rst (I only care about the link tag, as I'm tacking that regression). Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat) P.S.: As the Linux kernel's regression tracker I deal with a lot of reports and sometimes miss something important when writing mails like this. If that's the case here, don't hesitate to tell me in a public reply, it's in everyone's interest to set the public record straight.
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