Re: [PATCH v2 2/6] x86/mm/pat: Add pat_disable() interface

From: Borislav Petkov
Date: Tue Mar 22 2016 - 12:59:59 EST


On Wed, Mar 16, 2016 at 06:46:55PM -0600, Toshi Kani wrote:
> In preparation to fix a regression caused by 'commit 9cd25aac1f44
> ("x86/mm/pat: Emulate PAT when it is disabled")', PAT needs to
> provide an interface that disables the OS to initialize PAT MSR.

prevents the OS from initializing the PAT MSR.

>
> PAT MSR initialization must be done on all CPUs with the specific

s/with/using/

> sequence of operations defined in Intel SDM. This requires MTRR
^
the

s/MTRR/MTRRs/

> to be enabled since pat_init() is called as part of MTRR init
> from mtrr_rendezvous_handler().
>
> Change pat_disable() as the interface to disable the OS to initialize
> PAT MSR, and set PAT table with pat_keep_handoff_state(). This
> interface can be called when PAT initialization may not be performed.

This paragraph reads funky and I can't really parse what it is trying to
say.

> This also assures that pat_disable() called from pat_bsp_init()
> to set PAT table properly when CPU does not support PAT.
>
> Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Luis R. Rodriguez <mcgrof@xxxxxxxx>
> Cc: Juergen Gross <jgross@xxxxxxxx>
> Cc: Robert Elliott <elliott@xxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
> arch/x86/include/asm/pat.h | 1 +
> arch/x86/mm/pat.c | 21 ++++++++++++++++++---
> 2 files changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/pat.h b/arch/x86/include/asm/pat.h
> index ca6c228..016142b 100644
> --- a/arch/x86/include/asm/pat.h
> +++ b/arch/x86/include/asm/pat.h
> @@ -5,6 +5,7 @@
> #include <asm/pgtable_types.h>
>
> bool pat_enabled(void);
> +void pat_disable(const char *reason);
> extern void pat_init(void);
> void pat_init_cache_modes(u64);
>
> diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
> index e0a34b0..48d1619 100644
> --- a/arch/x86/mm/pat.c
> +++ b/arch/x86/mm/pat.c
> @@ -40,11 +40,26 @@
> static bool boot_cpu_done;
>
> static int __read_mostly __pat_enabled = IS_ENABLED(CONFIG_X86_PAT);
> +static void pat_keep_handoff_state(void);
>
> -static inline void pat_disable(const char *reason)
> +/**
> + * pat_disable() - Disable the OS to initialize PAT MSR

^^^^

Err, what? The function name can't be more clear.

> + *
> + * This function disables the OS to initialize PAT MSR, and calls

"prevents the OS from initializing the PAT MSR..."

> + * pat_keep_handoff_state() to set PAT table to the handoff state.

We can see what is calls. You're explaining *what* the code does instead
of *why* again.

> + */
> +void pat_disable(const char *reason)
> {

Why aren't you checking __pat_enabled here?

if (!__pat_enabled)
return;

You can save yourself the other guards in that function, especially that
pr_err() below.

> + if (boot_cpu_done) {
> + pr_err("x86/PAT: PAT cannot be disabled after initialization "
> + "(attempting: %s)\n", reason);

Please integrate checkpatch.pl into your patch creation workflow as it
sometimes has valid complaints:

WARNING: quoted string split across lines
#79: FILE: arch/x86/mm/pat.c:55:
+ pr_err("x86/PAT: PAT cannot be disabled after initialization "
+ "(attempting: %s)\n", reason);

More to the point: why do we need that pr_err() call? What is that
supposed to tell the user?

I think it is more for the programmer to catch wrong use of
pat_disable() and then it should be WARN_ONCE() or so...

> + return;
> + }
> +
> __pat_enabled = 0;
> pr_info("x86/PAT: %s\n", reason);
> +
> + pat_keep_handoff_state();
> }
>
> static int __init nopat(char *str)
> @@ -202,7 +217,7 @@ static void pat_bsp_init(u64 pat)
> {
> u64 tmp_pat;
>
> - if (!cpu_has_pat) {
> + if (!boot_cpu_has(X86_FEATURE_PAT)) {
> pat_disable("PAT not supported by CPU.");
> return;
> }
> @@ -220,7 +235,7 @@ static void pat_bsp_init(u64 pat)
>
> static void pat_ap_init(u64 pat)
> {
> - if (!cpu_has_pat) {
> + if (!boot_cpu_has(X86_FEATURE_PAT)) {
> /*
> * If this happens we are on a secondary CPU, but switched to
> * PAT on the boot CPU. We have no way to undo PAT.

Those last two hunks are unrelated changes and should be a separate
patch.

--
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
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