Messages in this thread Patch in this message | | | Date | Thu, 17 Aug 2023 12:15:13 +0000 | Subject | [PATCH RESEND] x86/entry: Don't write to CR3 when restoring to kernel CR3 | From | Brendan Jackman <> |
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From: Lai Jiangshan <laijs@linux.alibaba.com>
Skip resuming KERNEL pages since it is already KERNEL CR3
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com> Signed-off-by: Brendan Jackman <jackmanb@google.com> ---
While staring at paranoid_exit I was confused about why we had this CR3 write, avoiding it seems like a free optimisation. The original commit 21e94459110252 ("x86/mm: Optimize RESTORE_CR3") says "Most NMI/paranoid exceptions will not in fact change pagetables" but I didn't't understand what the "most" was referring to. I then discovered this patch on the mailing list, Andy said[1] that it looks correct so maybe now is the time to merge it?
Note there's another patch in [1] as well, the benefit of that one is not obvious to me though.
We've tested an equivalent patch in our internal kernel.
[1] https://lore.kernel.org/lkml/20200526043507.51977-3-laijs@linux.alibaba.com/ -- >8 -- arch/x86/entry/calling.h | 13 ++++--------- 1 file changed, 4 insertions(+), 9 deletions(-)
diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h index f6907627172b..b2458685d56e 100644 --- a/arch/x86/entry/calling.h +++ b/arch/x86/entry/calling.h @@ -236,14 +236,13 @@ For 32-bit we have the following conventions - kernel is built with .macro RESTORE_CR3 scratch_reg:req save_reg:req ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI - ALTERNATIVE "jmp .Lwrcr3_\@", "", X86_FEATURE_PCID - /* - * KERNEL pages can always resume with NOFLUSH as we do - * explicit flushes. + * Skip resuming KERNEL pages since it is already KERNEL CR3. */ bt $PTI_USER_PGTABLE_BIT, \save_reg - jnc .Lnoflush_\@ + jnc .Lend_\@ + + ALTERNATIVE "jmp .Lwrcr3_\@", "", X86_FEATURE_PCID /* * Check if there's a pending flush for the user ASID we're @@ -261,10 +260,6 @@ For 32-bit we have the following conventions - kernel is built with SET_NOFLUSH_BIT \save_reg .Lwrcr3_\@: - /* - * The CR3 write could be avoided when not changing its value, - * but would require a CR3 read *and* a scratch register. - */ movq \save_reg, %cr3 .Lend_\@: .endm -- 2.41.0.694.ge786442a9b-goog
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