From: Borislav Petkov <bp@alien8.de>
To: Alexey Kardashevskiy <aik@amd.com>
Cc: kvm@vger.kernel.org, x86@kernel.org,
linux-kernel@vger.kernel.org,
Venu Busireddy <venu.busireddy@oracle.com>,
Tony Luck <tony.luck@intel.com>,
Tom Lendacky <thomas.lendacky@amd.com>,
Thomas Gleixner <tglx@linutronix.de>,
Sean Christopherson <seanjc@google.com>,
Peter Zijlstra <peterz@infradead.org>,
Paolo Bonzini <pbonzini@redhat.com>,
Michael Sterritt <sterritt@google.com>,
Michael Roth <michael.roth@amd.com>,
Mario Limonciello <mario.limonciello@amd.com>,
Ingo Molnar <mingo@redhat.com>,
Heiko Carstens <hca@linux.ibm.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Dave Hansen <dave.hansen@linux.intel.com>,
Andrew Cooper <andrew.cooper3@citrix.com>,
"Jason A. Donenfeld" <Jason@zx2c4.com>,
"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH kernel 1/3] x86/amd/dr_addr_mask: Cache values in percpu variables
Date: Wed, 7 Dec 2022 19:55:57 +0100 [thread overview]
Message-ID: <Y5DhvRZX0Aizu1ya@zn.tnic> (raw)
In-Reply-To: <20221201021948.9259-2-aik@amd.com>
On Thu, Dec 01, 2022 at 01:19:46PM +1100, Alexey Kardashevskiy wrote:
> Subject: Re: [PATCH kernel 1/3] x86/amd/dr_addr_mask: Cache values in percpu variables
"x86/amd: " is perfectly fine as a prefix.
> Reading DR[0-3]_ADDR_MASK MSRs takes about 250 cycles which is going to
> be noticeable when the AMD KVM SEV-ES's DebugSwap feature is enabled and
which does what? I.e., a sort of lazy DR regs swapping...
> KVM needs to store these before switching to a guest; the DebugSwitch
> hardware support restores them as type B swap.
I know this is all clear to you but you should explain what type B
register swap is.
> This stores MSR values from set_dr_addr_mask() in percpu values and
s/This stores/Store/
From Documentation/process/submitting-patches.rst:
"Describe your changes in imperative mood, e.g. "make xyzzy do frotz"
instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy
to do frotz", as if you are giving orders to the codebase to change
its behaviour."
Also, do not talk about what your patch does - that should hopefully be
visible in the diff itself. Rather, talk about *why* you're doing what
you're doing.
> returns them via new get_dr_addr_mask(). The gain here is about 10x.
>
> Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
> ---
> arch/x86/include/asm/debugreg.h | 1 +
> arch/x86/kernel/cpu/amd.c | 32 ++++++++++++++++++++
> 2 files changed, 33 insertions(+)
>
> diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
> index cfdf307ddc01..c4324d0205b5 100644
> --- a/arch/x86/include/asm/debugreg.h
> +++ b/arch/x86/include/asm/debugreg.h
> @@ -127,6 +127,7 @@ static __always_inline void local_db_restore(unsigned long dr7)
>
> #ifdef CONFIG_CPU_SUP_AMD
> extern void set_dr_addr_mask(unsigned long mask, int dr);
> +extern unsigned long get_dr_addr_mask(int dr);
> #else
> static inline void set_dr_addr_mask(unsigned long mask, int dr) { }
> #endif
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index c75d75b9f11a..ec7efcef4e14 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -1158,6 +1158,11 @@ static bool cpu_has_amd_erratum(struct cpuinfo_x86 *cpu, const int *erratum)
> return false;
> }
>
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr0_addr_mask);
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr1_addr_mask);
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr2_addr_mask);
> +DEFINE_PER_CPU_READ_MOSTLY(unsigned long, dr3_addr_mask);
This BPEXT thing is AMD-only, right?
I guess those should be called amd_drX_addr_mask where X in [0-3].
Yeah yeah, they are used in AMD-only code - svm* - but still.
> void set_dr_addr_mask(unsigned long mask, int dr)
> {
> if (!boot_cpu_has(X86_FEATURE_BPEXT))
> @@ -1166,17 +1171,44 @@ void set_dr_addr_mask(unsigned long mask, int dr)
> switch (dr) {
> case 0:
> wrmsr(MSR_F16H_DR0_ADDR_MASK, mask, 0);
> + per_cpu(dr0_addr_mask, smp_processor_id()) = mask;
> break;
> case 1:
> + wrmsr(MSR_F16H_DR1_ADDR_MASK - 1 + dr, mask, 0);
> + per_cpu(dr1_addr_mask, smp_processor_id()) = mask;
> + break;
> case 2:
> + wrmsr(MSR_F16H_DR1_ADDR_MASK - 1 + dr, mask, 0);
> + per_cpu(dr2_addr_mask, smp_processor_id()) = mask;
> + break;
> case 3:
> wrmsr(MSR_F16H_DR1_ADDR_MASK - 1 + dr, mask, 0);
> + per_cpu(dr3_addr_mask, smp_processor_id()) = mask;
> break;
> default:
> break;
> }
> }
>
> +unsigned long get_dr_addr_mask(int dr)
This function name is too generic for an exported function.
amd_get_dr_addr_mask() I'd say.
> + if (!boot_cpu_has(X86_FEATURE_BPEXT))
check_for_deprecated_apis: WARNING: arch/x86/kernel/cpu/amd.c:1195: Do not use boot_cpu_has() - use cpu_feature_enabled() instead.
You could fix the above one too, while at it.
> + return 0;
> +
> + switch (dr) {
> + case 0:
> + return per_cpu(dr0_addr_mask, smp_processor_id());
> + case 1:
> + return per_cpu(dr1_addr_mask, smp_processor_id());
> + case 2:
> + return per_cpu(dr2_addr_mask, smp_processor_id());
> + case 3:
> + return per_cpu(dr3_addr_mask, smp_processor_id());
default:
WARN_ON_ONCE(1);
break;
Just in case.
And as a matter of fact, make that short and succinct:
switch (dr) {
case 0: return per_cpu(dr0_addr_mask, smp_processor_id());
case 1: return per_cpu(dr1_addr_mask, smp_processor_id());
case 2: return per_cpu(dr2_addr_mask, smp_processor_id());
case 3: return per_cpu(dr3_addr_mask, smp_processor_id());
default: WARN_ON_ONCE(1); break;
}
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2022-12-07 18:56 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-01 2:19 [PATCH kernel 0/3] KVM: SEV: Enable AMD SEV-ES DebugSwap Alexey Kardashevskiy
2022-12-01 2:19 ` [PATCH kernel 1/3] x86/amd/dr_addr_mask: Cache values in percpu variables Alexey Kardashevskiy
2022-12-01 16:58 ` Sean Christopherson
2022-12-01 19:45 ` Andrew Cooper
2022-12-06 7:14 ` Alexey Kardashevskiy
2022-12-06 17:07 ` Sean Christopherson
2022-12-07 0:50 ` Alexey Kardashevskiy
2022-12-07 18:55 ` Borislav Petkov [this message]
2022-12-08 6:11 ` Alexey Kardashevskiy
2022-12-08 10:35 ` Borislav Petkov
2022-12-01 2:19 ` [PATCH kernel 2/3] KVM: SEV: Enable DebugSwap Alexey Kardashevskiy
2022-12-01 17:37 ` Sean Christopherson
2022-12-09 2:28 ` Alexey Kardashevskiy
2022-12-01 2:19 ` [PATCH kernel 3/3] x86/sev: Do not handle #VC for DR7 read/write Alexey Kardashevskiy
2022-12-01 17:38 ` Sean Christopherson
2022-12-07 19:01 ` Borislav Petkov
2022-12-07 19:07 ` Sean Christopherson
2022-12-08 7:14 ` Alexey Kardashevskiy
2022-12-08 11:01 ` Borislav Petkov
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