diff mbox series

[v2,1/3] arch/arm64: Introduce a capability to tell whether 16-bit VMID is available

Message ID 20210616155606.2806-2-shameerali.kolothum.thodi@huawei.com (mailing list archive)
State New, archived
Headers show
Series kvm/arm: New VMID allocator based on asid | expand

Commit Message

Shameerali Kolothum Thodi June 16, 2021, 3:56 p.m. UTC 
From: Julien Grall <julien.grall@arm.com>

At the moment, the function kvm_get_vmid_bits() is looking up for the
sanitized value of ID_AA64MMFR1_EL1 and extract the information
regarding the number of VMID bits supported.

This is fine as the function is mainly used during VMID roll-over. New
use in a follow-up patch will require the function to be called a every
context switch so we want the function to be more efficient.

A new capability is introduced to tell whether 16-bit VMID is
available.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Shameer Kolothum <shameerali.kolothum.thodi@huawei.com>
---
 arch/arm64/include/asm/kvm_mmu.h | 4 +---
 arch/arm64/kernel/cpufeature.c   | 9 +++++++++
 arch/arm64/tools/cpucaps         | 1 +
 3 files changed, 11 insertions(+), 3 deletions(-)

Comments

Will Deacon July 21, 2021, 3:23 p.m. UTC | #1 
On Wed, Jun 16, 2021 at 04:56:04PM +0100, Shameer Kolothum wrote:
> From: Julien Grall <julien.grall@arm.com>
> 
> At the moment, the function kvm_get_vmid_bits() is looking up for the
> sanitized value of ID_AA64MMFR1_EL1 and extract the information
> regarding the number of VMID bits supported.
> 
> This is fine as the function is mainly used during VMID roll-over. New
> use in a follow-up patch will require the function to be called a every
> context switch so we want the function to be more efficient.
> 
> A new capability is introduced to tell whether 16-bit VMID is
> available.

I don't really buy this rationale. The VMID allocator introduced later on
caches this value in the static 'vmid_bits' variable, and that gets used
on vCPU enter via vmid_gen_match() in the kvm_arm_update_vmid() fastpath.

So I would prefer that we just expose an accessor for that than introduce
a static key and new cpufeature just for kvm_get_vttbr().

Will
Shameerali Kolothum Thodi July 22, 2021, 6:24 a.m. UTC | #2 
> -----Original Message-----
> From: Will Deacon [mailto:will@kernel.org]
> Sent: 21 July 2021 16:23
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
> Cc: linux-arm-kernel@lists.infradead.org; kvmarm@lists.cs.columbia.edu;
> linux-kernel@vger.kernel.org; maz@kernel.org; catalin.marinas@arm.com;
> james.morse@arm.com; julien.thierry.kdev@gmail.com;
> suzuki.poulose@arm.com; jean-philippe@linaro.org;
> Alexandru.Elisei@arm.com; Linuxarm <linuxarm@huawei.com>
> Subject: Re: [PATCH v2 1/3] arch/arm64: Introduce a capability to tell whether
> 16-bit VMID is available
> 
> On Wed, Jun 16, 2021 at 04:56:04PM +0100, Shameer Kolothum wrote:
> > From: Julien Grall <julien.grall@arm.com>
> >
> > At the moment, the function kvm_get_vmid_bits() is looking up for the
> > sanitized value of ID_AA64MMFR1_EL1 and extract the information
> > regarding the number of VMID bits supported.
> >
> > This is fine as the function is mainly used during VMID roll-over. New
> > use in a follow-up patch will require the function to be called a every
> > context switch so we want the function to be more efficient.
> >
> > A new capability is introduced to tell whether 16-bit VMID is
> > available.
> 
> I don't really buy this rationale. The VMID allocator introduced later on
> caches this value in the static 'vmid_bits' variable, and that gets used
> on vCPU enter via vmid_gen_match() in the kvm_arm_update_vmid() fastpath.
> 
> So I would prefer that we just expose an accessor for that than introduce
> a static key and new cpufeature just for kvm_get_vttbr().

Ok. Will change it to an accessor.

Thanks,
Shameer
diff mbox series

Patch

diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index 25ed956f9af1..2abdea7dcb43 100644
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -223,9 +223,7 @@  void kvm_toggle_cache(struct kvm_vcpu *vcpu, bool was_enabled);
 
 static inline unsigned int kvm_get_vmid_bits(void)
 {
-	int reg = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
-
-	return get_vmid_bits(reg);
+	return cpus_have_const_cap(ARM64_HAS_16BIT_VMID) ? 16 : 8;
 }
 
 /*
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index efed2830d141..0642ebe118b0 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c
@@ -2218,6 +2218,15 @@  static const struct arm64_cpu_capabilities arm64_features[] = {
 		.matches = has_cpuid_feature,
 		.min_field_value = 1,
 	},
+	{
+		.capability = ARM64_HAS_16BIT_VMID,
+		.type = ARM64_CPUCAP_SYSTEM_FEATURE,
+		.sys_reg = SYS_ID_AA64MMFR1_EL1,
+		.field_pos = ID_AA64MMFR1_VMIDBITS_SHIFT,
+		.sign = FTR_UNSIGNED,
+		.min_field_value = ID_AA64MMFR1_VMIDBITS_16,
+		.matches = has_cpuid_feature,
+	},
 	{},
 };
 
diff --git a/arch/arm64/tools/cpucaps b/arch/arm64/tools/cpucaps
index 21fbdda7086e..456b4cf1ba27 100644
--- a/arch/arm64/tools/cpucaps
+++ b/arch/arm64/tools/cpucaps
@@ -3,6 +3,7 @@ 
 # Internal CPU capabilities constants, keep this list sorted
 
 BTI
+HAS_16BIT_VMID
 HAS_32BIT_EL0
 HAS_32BIT_EL1
 HAS_ADDRESS_AUTH