Messages in this thread | | | Date | Fri, 15 Jul 2022 06:22:52 +0200 | From | Juergen Gross <> | Subject | Re: [PATCH v2] Subject: x86/PAT: Report PAT on CPUs that support PAT without MTRR |
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On 15.07.22 04:19, Chuck Zmudzinski wrote: > On 7/14/2022 1:40 AM, Juergen Gross wrote: >> 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") >>> Co-developed-by: Jan Beulich <jbeulich@suse.com> >>> Cc: stable@vger.kernel.org >>> Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com> >>> --- >>> v2: *Add force_pat_disabled variable to fix "nopat" on Xen PV (Jan Beulich) >>> *Add the necessary code to incorporate the "nopat" fix >>> *void init_cache_modes(void) -> void __init init_cache_modes(void) >>> *Add Jan Beulich as Co-developer (Jan has not signed off yet) >>> *Expand the commit message to include relevant parts of the commit >>> message of Jan Beulich's proposed patch for this problem >>> *Fix 'else if ... {' placement and indentation >>> *Remove indication the backport to stable branches is only back to 5.17.y >>> >>> I think these changes address all the comments on the original patch >>> >>> I added Jan Beulich as a Co-developer because Juergen Gross asked me to >>> include Jan's idea for fixing "nopat" that was missing from the first >>> version of the patch. >>> >>> The patch has been tested, it works as expected with and without nopat >>> in the Xen PV Dom0 environment. That is, "nopat" causes the system to >>> exhibit the effects and problems that lack of PAT support causes. >>> >>> arch/x86/mm/pat/memtype.c | 16 ++++++++++++++-- >>> 1 file changed, 14 insertions(+), 2 deletions(-) >>> >>> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c >>> index d5ef64ddd35e..10a37d309d23 100644 >>> --- a/arch/x86/mm/pat/memtype.c >>> +++ b/arch/x86/mm/pat/memtype.c >>> @@ -62,6 +62,7 @@ >>> >>> static bool __read_mostly pat_bp_initialized; >>> static bool __read_mostly pat_disabled = !IS_ENABLED(CONFIG_X86_PAT); >>> +static bool __initdata pat_force_disabled = !IS_ENABLED(CONFIG_X86_PAT); >>> static bool __read_mostly pat_bp_enabled; >>> static bool __read_mostly pat_cm_initialized; >>> >>> @@ -86,6 +87,7 @@ void pat_disable(const char *msg_reason) >>> static int __init nopat(char *str) >>> { >>> pat_disable("PAT support disabled via boot option."); >>> + pat_force_disabled = true; >>> return 0; >>> } >>> early_param("nopat", nopat); >>> @@ -272,7 +274,7 @@ static void pat_ap_init(u64 pat) >>> wrmsrl(MSR_IA32_CR_PAT, pat); >>> } >>> >>> -void init_cache_modes(void) >>> +void __init init_cache_modes(void) >>> { >>> u64 pat = 0; >>> >>> @@ -292,7 +294,7 @@ void init_cache_modes(void) >>> rdmsrl(MSR_IA32_CR_PAT, pat); >>> } >>> >>> - if (!pat) { >>> + if (!pat || pat_force_disabled) { >> >> Can we just remove this modification and ... >> >>> /* >>> * No PAT. Emulate the PAT table that corresponds to the two >>> * cache bits, PWT (Write Through) and PCD (Cache Disable). >>> @@ -313,6 +315,16 @@ void init_cache_modes(void) >>> */ >>> pat = PAT(0, WB) | PAT(1, WT) | PAT(2, UC_MINUS) | PAT(3, UC) | >>> PAT(4, WB) | PAT(5, WT) | PAT(6, UC_MINUS) | PAT(7, UC); >>> + } else if (!pat_bp_enabled) { >> >> ... use >> >> + } else if (!pat_bp_enabled && !pat_force_disabled) { >> >> here? >> >> This will result in the desired outcome in all cases IMO: If PAT wasn't >> disabled via "nopat" and the PAT MSR has a non-zero value (from BIOS or >> Hypervisor) and PAT has been disabled implicitly (e.g. due to lack of >> MTRR), then PAT will be set to "enabled" again. > > With that, you can also completely remove the new Boolean - it > will be a meaningless variable wasting memory. This will also make
No, it is making a difference with "nopat" having been specified.
In the Xen PV case we will have pat_bp_enabled == false due to the lack of MTRR. We don't want to set it to true if "nopat" has been specified on the command line, so pat_force_disabled should not be true when we are setting pat_bp_enabled to true again.
> my patch more or less do what Jan's patch does - the "nopat" option > will not cause the situation when the PAT MSR does not match the > software view. So you are basically proposing just going back to > my original patch, after fixing the style problems, of course. That > also would solve the problem of needing Jan's S-o-b. I am inclined, > however, to wait for a maintainer who has power to actually do the > commit, to make a comment. Your R-b to my v2 did not have much clout > with the actual maintainers, as far as I can tell. I am somewhat annoyed > that it was at your suggestion that my v2 ended up confusing the > main issue, the regression, with the red herring of the "nopat" > option.
I'm sorry for that.
Juergen [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |