[PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Gabriel Krisman Bertazi]

Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
in the hypervisor.  In fact, ARM documentation mentions some feature
registers are not supposed to be accessed frequently by the OS, and
therefore should be emulated for guests [1].

Commit 0388f9c74330 ("arm64: mm: Implement
arch_wants_old_prefaulted_pte()") introduced a read of this register in
the page fault path.  But, even when the feature of setting faultaround
pages with the old flag is disabled for a given cpu, we are still paying
the cost of checking the register on every pagefault. This results in an
explosion of vmexit events in KVM guests, which directly impacts the
performance of virtualized workloads.  For instance, running kernbench
yields a 15% increase in system time solely due to the increased vmexit
cycles.

This patch avoids the extra cost by using the sanitized cached value.
It should be safe to do so, since this register mustn't change for a
given cpu.

[1] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Learn%20the%20Architecture/Armv8-A%20virtualization.pdf?revision=a765a7df-1a00-434d-b241-357bfda2dd31

Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>

---
Changes since v1:
  - Rely on read_sanitised_ftr_reg instead of static var (will)
---
 arch/arm64/include/asm/cpufeature.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
index 03d1c9d7af82..3d1a5677321e 100644
--- a/arch/arm64/include/asm/cpufeature.h
+++ b/arch/arm64/include/asm/cpufeature.h
@@ -864,7 +864,11 @@ static inline bool cpu_has_hw_af(void)
 	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
 		return false;
 
-	mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);
+	/*
+	 * Use cached version to avoid emulated msr operation on KVM
+	 * guests.
+	 */
+	mmfr1 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
 	return cpuid_feature_extract_unsigned_field(mmfr1,
 						ID_AA64MMFR1_EL1_HAFDBS_SHIFT);
 }
-- 
2.35.3


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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Anshuman Khandual]

On 1/9/23 20:49, Gabriel Krisman Bertazi wrote:
> Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
> in the hypervisor.  In fact, ARM documentation mentions some feature
> registers are not supposed to be accessed frequently by the OS, and
> therefore should be emulated for guests [1].

I am just curious, is this the only system register access (AA64MMFR1_EL1)
causing such performance problems ?

> 
> Commit 0388f9c74330 ("arm64: mm: Implement
> arch_wants_old_prefaulted_pte()") introduced a read of this register in
> the page fault path.  But, even when the feature of setting faultaround

Right, although cpu_has_hw_af() was added earlier via commit 47d7b15b88f9
("arm64: cpufeature: introduce helper cpu_has_hw_af()"), but above commit
did add this on regular page fault path via do_set_pte().


> pages with the old flag is disabled for a given cpu, we are still paying
> the cost of checking the register on every pagefault. This results in an

Right, accessing a system register on each page fault is not optimal.

> explosion of vmexit events in KVM guests, which directly impacts the
> performance of virtualized workloads.  For instance, running kernbench
> yields a 15% increase in system time solely due to the increased vmexit
> cycles.
> 
> This patch avoids the extra cost by using the sanitized cached value.
> It should be safe to do so, since this register mustn't change for a
> given cpu.
> 
> [1] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Learn%20the%20Architecture/Armv8-A%20virtualization.pdf?revision=a765a7df-1a00-434d-b241-357bfda2dd31
> 
> Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
> 
> ---
> Changes since v1:
>   - Rely on read_sanitised_ftr_reg instead of static var (will)
> ---
>  arch/arm64/include/asm/cpufeature.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 03d1c9d7af82..3d1a5677321e 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -864,7 +864,11 @@ static inline bool cpu_has_hw_af(void)
>  	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
>  		return false;
>  
> -	mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);
> +	/*
> +	 * Use cached version to avoid emulated msr operation on KVM
> +	 * guests.
> +	 */
> +	mmfr1 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
>  	return cpuid_feature_extract_unsigned_field(mmfr1,
>  						ID_AA64MMFR1_EL1_HAFDBS_SHIFT);
>  }

LGTM but as mentioned earlier, are there not other similar instances or this
is just more problematic being on direct page fault path ? 

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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Gabriel Krisman Bertazi]

Anshuman Khandual <anshuman.khandual@arm.com> writes:

> On 1/9/23 20:49, Gabriel Krisman Bertazi wrote:
>> Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
>> in the hypervisor.  In fact, ARM documentation mentions some feature
>> registers are not supposed to be accessed frequently by the OS, and
>> therefore should be emulated for guests [1].
>
> I am just curious, is this the only system register access (AA64MMFR1_EL1)
> causing such performance problems ?

I haven't audited all the system registers.  For AA64MMFR1_EL1 this is
the only instance where the frequency of access affects performance in a
meaningful way for my workload.

I have a real-world bug report about it, and by profiling vm exit
events, I can also argue this is the only instance of any emulated msr
read/write that happens frequently enough to change the order of
magnitude of exit events measured by perf for my workload between, at
least, 5.4 (it was introduced in v5.12, but I have data back to 5.4) and
mainline.

>> Commit 0388f9c74330 ("arm64: mm: Implement
>> arch_wants_old_prefaulted_pte()") introduced a read of this register in
>> the page fault path.  But, even when the feature of setting faultaround
>
> Right, although cpu_has_hw_af() was added earlier via commit 47d7b15b88f9
> ("arm64: cpufeature: introduce helper cpu_has_hw_af()"), but above commit
> did add this on regular page fault path via do_set_pte().

Indeed.  The only other usage of this function is in wp_page_copy, and,
from what I can tell, it is in an unlikely() branch when COW is being
performed on a page that was recently unmapped.  It is not something
frequent enough that I saw in profiling.

>>  						ID_AA64MMFR1_EL1_HAFDBS_SHIFT);
>>  }
>
> LGTM but as mentioned earlier, are there not other similar instances or this
> is just more problematic being on direct page fault path ?

I think a full audit of the emulated system registers in kvm will be
required to definitely answer it.  But this instance is, by far, the hottest
case in the codebase.

-- 
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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Anshuman Khandual]

On 1/11/23 19:01, Gabriel Krisman Bertazi wrote:
> Anshuman Khandual <anshuman.khandual@arm.com> writes:
> 
>> On 1/9/23 20:49, Gabriel Krisman Bertazi wrote:
>>> Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
>>> in the hypervisor.  In fact, ARM documentation mentions some feature
>>> registers are not supposed to be accessed frequently by the OS, and
>>> therefore should be emulated for guests [1].
>>
>> I am just curious, is this the only system register access (AA64MMFR1_EL1)
>> causing such performance problems ?
> 
> I haven't audited all the system registers.  For AA64MMFR1_EL1 this is
> the only instance where the frequency of access affects performance in a
> meaningful way for my workload.
> 
> I have a real-world bug report about it, and by profiling vm exit
> events, I can also argue this is the only instance of any emulated msr
> read/write that happens frequently enough to change the order of
> magnitude of exit events measured by perf for my workload between, at
> least, 5.4 (it was introduced in v5.12, but I have data back to 5.4) and
> mainline.
> 
>>> Commit 0388f9c74330 ("arm64: mm: Implement
>>> arch_wants_old_prefaulted_pte()") introduced a read of this register in
>>> the page fault path.  But, even when the feature of setting faultaround
>>
>> Right, although cpu_has_hw_af() was added earlier via commit 47d7b15b88f9
>> ("arm64: cpufeature: introduce helper cpu_has_hw_af()"), but above commit
>> did add this on regular page fault path via do_set_pte().
> 
> Indeed.  The only other usage of this function is in wp_page_copy, and,
> from what I can tell, it is in an unlikely() branch when COW is being
> performed on a page that was recently unmapped.  It is not something
> frequent enough that I saw in profiling.
> 
>>>  						ID_AA64MMFR1_EL1_HAFDBS_SHIFT);
>>>  }
>>
>> LGTM but as mentioned earlier, are there not other similar instances or this
>> is just more problematic being on direct page fault path ?
> 
> I think a full audit of the emulated system registers in kvm will be
> required to definitely answer it.  But this instance is, by far, the hottest
> case in the codebase.
> 

Thanks for the additional details.

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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Anshuman Khandual]

On 1/9/23 20:49, Gabriel Krisman Bertazi wrote:
> Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
> in the hypervisor.  In fact, ARM documentation mentions some feature
> registers are not supposed to be accessed frequently by the OS, and
> therefore should be emulated for guests [1].
> 
> Commit 0388f9c74330 ("arm64: mm: Implement
> arch_wants_old_prefaulted_pte()") introduced a read of this register in
> the page fault path.  But, even when the feature of setting faultaround
> pages with the old flag is disabled for a given cpu, we are still paying
> the cost of checking the register on every pagefault. This results in an
> explosion of vmexit events in KVM guests, which directly impacts the
> performance of virtualized workloads.  For instance, running kernbench
> yields a 15% increase in system time solely due to the increased vmexit
> cycles.
> 
> This patch avoids the extra cost by using the sanitized cached value.
> It should be safe to do so, since this register mustn't change for a
> given cpu.
> 
> [1] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Learn%20the%20Architecture/Armv8-A%20virtualization.pdf?revision=a765a7df-1a00-434d-b241-357bfda2dd31

A small nit. This URL length causes a checkpatch.pl warning.

WARNING: Possible unwrapped commit description (prefer a maximum 75 chars per line)

Just wondering, if this URL can be replaced with a document identification
number or something similar for a more cleaner commit message, not sure if
such a thing is even feasible.

> 
> Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
> 
> ---
> Changes since v1:
>   - Rely on read_sanitised_ftr_reg instead of static var (will)
> ---
>  arch/arm64/include/asm/cpufeature.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 03d1c9d7af82..3d1a5677321e 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -864,7 +864,11 @@ static inline bool cpu_has_hw_af(void)
>  	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
>  		return false;
>  
> -	mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);
> +	/*
> +	 * Use cached version to avoid emulated msr operation on KVM
> +	 * guests.
> +	 */
> +	mmfr1 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
>  	return cpuid_feature_extract_unsigned_field(mmfr1,
>  						ID_AA64MMFR1_EL1_HAFDBS_SHIFT);
>  }

FWIW

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>

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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Gabriel Krisman Bertazi]

Gabriel Krisman Bertazi <krisman@suse.de> writes:

> Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
> in the hypervisor.  In fact, ARM documentation mentions some feature
> registers are not supposed to be accessed frequently by the OS, and
> therefore should be emulated for guests [1].
>
> Commit 0388f9c74330 ("arm64: mm: Implement
> arch_wants_old_prefaulted_pte()") introduced a read of this register in
> the page fault path.  But, even when the feature of setting faultaround
> pages with the old flag is disabled for a given cpu, we are still paying
> the cost of checking the register on every pagefault. This results in an
> explosion of vmexit events in KVM guests, which directly impacts the
> performance of virtualized workloads.  For instance, running kernbench
> yields a 15% increase in system time solely due to the increased vmexit
> cycles.
>
> This patch avoids the extra cost by using the sanitized cached value.
> It should be safe to do so, since this register mustn't change for a
> given cpu.
>
> [1] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Learn%20the%20Architecture/Armv8-A%20virtualization.pdf?revision=a765a7df-1a00-434d-b241-357bfda2dd31
>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>

Hi,

Considering the performance impact on kvm guests, unless someone
opposes, can we get this queued already for -rc5?

-- 
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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Will Deacon]

On Mon, Jan 16, 2023 at 05:41:35PM -0300, Gabriel Krisman Bertazi wrote:
> Gabriel Krisman Bertazi <krisman@suse.de> writes:
> 
> > Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
> > in the hypervisor.  In fact, ARM documentation mentions some feature
> > registers are not supposed to be accessed frequently by the OS, and
> > therefore should be emulated for guests [1].
> >
> > Commit 0388f9c74330 ("arm64: mm: Implement
> > arch_wants_old_prefaulted_pte()") introduced a read of this register in
> > the page fault path.  But, even when the feature of setting faultaround
> > pages with the old flag is disabled for a given cpu, we are still paying
> > the cost of checking the register on every pagefault. This results in an
> > explosion of vmexit events in KVM guests, which directly impacts the
> > performance of virtualized workloads.  For instance, running kernbench
> > yields a 15% increase in system time solely due to the increased vmexit
> > cycles.
> >
> > This patch avoids the extra cost by using the sanitized cached value.
> > It should be safe to do so, since this register mustn't change for a
> > given cpu.
> >
> > [1] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Learn%20the%20Architecture/Armv8-A%20virtualization.pdf?revision=a765a7df-1a00-434d-b241-357bfda2dd31
> >
> > Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
> 
> Hi,
> 
> Considering the performance impact on kvm guests, unless someone
> opposes, can we get this queued already for -rc5?

Given this has been the case since v5.12 afaict and it's not a correctness
issue, I was thinking we could queue this for 6.3?

Will

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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Gabriel Krisman Bertazi]

Will Deacon <will@kernel.org> writes:

> On Mon, Jan 16, 2023 at 05:41:35PM -0300, Gabriel Krisman Bertazi wrote:
>> 
>> Considering the performance impact on kvm guests, unless someone
>> opposes, can we get this queued already for -rc5?
>
> Given this has been the case since v5.12 afaict and it's not a correctness
> issue, I was thinking we could queue this for 6.3?

hey Will,

On a second thought, I'm fine with having it only in 6.3. :)


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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Catalin Marinas]

On Mon, Jan 09, 2023 at 12:19:55PM -0300, Gabriel Krisman Bertazi wrote:
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 03d1c9d7af82..3d1a5677321e 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -864,7 +864,11 @@ static inline bool cpu_has_hw_af(void)
>  	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
>  		return false;
>  
> -	mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);
> +	/*
> +	 * Use cached version to avoid emulated msr operation on KVM
> +	 * guests.
> +	 */
> +	mmfr1 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
>  	return cpuid_feature_extract_unsigned_field(mmfr1,
>  						ID_AA64MMFR1_EL1_HAFDBS_SHIFT);
>  }

You can probably make it slightly faster if you add a cpucap feature. We
have ARM64_HW_DBM but not ARM64_HW_AF. Not sure you'd notice a
difference in practice though.

-- 
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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Gabriel Krisman Bertazi]

Catalin Marinas <catalin.marinas@arm.com> writes:

> On Mon, Jan 09, 2023 at 12:19:55PM -0300, Gabriel Krisman Bertazi wrote:
>> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
>> index 03d1c9d7af82..3d1a5677321e 100644
>> --- a/arch/arm64/include/asm/cpufeature.h
>> +++ b/arch/arm64/include/asm/cpufeature.h
>> @@ -864,7 +864,11 @@ static inline bool cpu_has_hw_af(void)
>>  	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
>>  		return false;
>>  
>> -	mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);
>> +	/*
>> +	 * Use cached version to avoid emulated msr operation on KVM
>> +	 * guests.
>> +	 */
>> +	mmfr1 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
>>  	return cpuid_feature_extract_unsigned_field(mmfr1,
>>  						ID_AA64MMFR1_EL1_HAFDBS_SHIFT);
>>  }
>
> You can probably make it slightly faster if you add a cpucap feature. We
> have ARM64_HW_DBM but not ARM64_HW_AF. Not sure you'd notice a
> difference in practice though.

Hi Catalin,

I had a patch for that, but when I noticed the existence of
ARM64_HW_DBM, I dropped it. it is mentioned quickly in the v1.

From my measurements, when comparing this version to a static variable
caching the value (patch v1), the bsearch performance impact was not
significant.  I think it would be fine to keep this version.

but, of course, I can add the cpucap bit if it is preferred.  Please,
let me know.

Thanks,

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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Will Deacon]

On Mon, Jan 09, 2023 at 12:19:55PM -0300, Gabriel Krisman Bertazi wrote:
> Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
> in the hypervisor.  In fact, ARM documentation mentions some feature
> registers are not supposed to be accessed frequently by the OS, and
> therefore should be emulated for guests [1].
> 
> Commit 0388f9c74330 ("arm64: mm: Implement
> arch_wants_old_prefaulted_pte()") introduced a read of this register in
> the page fault path.  But, even when the feature of setting faultaround
> pages with the old flag is disabled for a given cpu, we are still paying
> the cost of checking the register on every pagefault. This results in an
> explosion of vmexit events in KVM guests, which directly impacts the
> performance of virtualized workloads.  For instance, running kernbench
> yields a 15% increase in system time solely due to the increased vmexit
> cycles.
> 
> This patch avoids the extra cost by using the sanitized cached value.
> It should be safe to do so, since this register mustn't change for a
> given cpu.
> 
> [1] https://developer.arm.com/-/media/Arm%20Developer%20Community/PDF/Learn%20the%20Architecture/Armv8-A%20virtualization.pdf?revision=a765a7df-1a00-434d-b241-357bfda2dd31
> 
> Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
> 
> ---
> Changes since v1:
>   - Rely on read_sanitised_ftr_reg instead of static var (will)
> ---
>  arch/arm64/include/asm/cpufeature.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 03d1c9d7af82..3d1a5677321e 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -864,7 +864,11 @@ static inline bool cpu_has_hw_af(void)
>  	if (!IS_ENABLED(CONFIG_ARM64_HW_AFDBM))
>  		return false;
>  
> -	mmfr1 = read_cpuid(ID_AA64MMFR1_EL1);
> +	/*
> +	 * Use cached version to avoid emulated msr operation on KVM
> +	 * guests.
> +	 */
> +	mmfr1 = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
>  	return cpuid_feature_extract_unsigned_field(mmfr1,
>  						ID_AA64MMFR1_EL1_HAFDBS_SHIFT);
>  }

Acked-by: Will Deacon <will@kernel.org>

Will

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Re: [PATCH v2] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path

[Catalin Marinas]

On Mon, 09 Jan 2023 12:19:55 -0300, Gabriel Krisman Bertazi wrote:
> Accessing AA64MMFR1_EL1 is expensive in KVM guests, since it is emulated
> in the hypervisor.  In fact, ARM documentation mentions some feature
> registers are not supposed to be accessed frequently by the OS, and
> therefore should be emulated for guests [1].
> 
> Commit 0388f9c74330 ("arm64: mm: Implement
> arch_wants_old_prefaulted_pte()") introduced a read of this register in
> the page fault path.  But, even when the feature of setting faultaround
> pages with the old flag is disabled for a given cpu, we are still paying
> the cost of checking the register on every pagefault. This results in an
> explosion of vmexit events in KVM guests, which directly impacts the
> performance of virtualized workloads.  For instance, running kernbench
> yields a 15% increase in system time solely due to the increased vmexit
> cycles.
> 
> [...]

Applied to arm64 (for-next/misc), thanks!

[1/1] arm64: Avoid repeated AA64MMFR1_EL1 register read on pagefault path
      https://git.kernel.org/arm64/c/a89c6bcdac22

-- 
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