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From: Marc Zyngier <maz@kernel.org>
To: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Cc: James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Quentin Perret <qperret@google.com>,
	Will Deacon <will@kernel.org>,
	broonie@kernel.org, kernel-team@android.com
Subject: [PATCH 4/4] arm64/fpsimd: Document the use of TIF_FOREIGN_FPSTATE by KVM
Date: Thu, 21 Oct 2021 16:11:24 +0100	[thread overview]
Message-ID: <20211021151124.3098113-5-maz@kernel.org> (raw)
In-Reply-To: <20211021151124.3098113-1-maz@kernel.org>

The bit of documentation that talks about TIF_FOREIGN_FPSTATE
does not mention the ungodly tricks that KVM plays with this flag.

Try and document this for the posterity.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kernel/fpsimd.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index ff4962750b3d..65af2ed64c6a 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -78,7 +78,10 @@
  * indicate whether or not the userland FPSIMD state of the current task is
  * present in the registers. The flag is set unless the FPSIMD registers of this
  * CPU currently contain the most recent userland FPSIMD state of the current
- * task.
+ * task *or* the state of the corresponding KVM vcpu if userspace is behaving
+ * as a VMM and that the vcpu has used FP during its last run. In the latter
+ * case, KVM will set TIF_FOREIGN_FPSTATE on kvm_vcpu_put(). For all intents
+ * and purposes, the vcpu FP state is treated identically to userspace's.
  *
  * In order to allow softirq handlers to use FPSIMD, kernel_neon_begin() may
  * save the task's FPSIMD context back to task_struct from softirq context.
-- 
2.30.2


WARNING: multiple messages have this Message-ID (diff)
From: Marc Zyngier <maz@kernel.org>
To: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Cc: kernel-team@android.com, Will Deacon <will@kernel.org>,
	broonie@kernel.org
Subject: [PATCH 4/4] arm64/fpsimd: Document the use of TIF_FOREIGN_FPSTATE by KVM
Date: Thu, 21 Oct 2021 16:11:24 +0100	[thread overview]
Message-ID: <20211021151124.3098113-5-maz@kernel.org> (raw)
In-Reply-To: <20211021151124.3098113-1-maz@kernel.org>

The bit of documentation that talks about TIF_FOREIGN_FPSTATE
does not mention the ungodly tricks that KVM plays with this flag.

Try and document this for the posterity.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kernel/fpsimd.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index ff4962750b3d..65af2ed64c6a 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -78,7 +78,10 @@
  * indicate whether or not the userland FPSIMD state of the current task is
  * present in the registers. The flag is set unless the FPSIMD registers of this
  * CPU currently contain the most recent userland FPSIMD state of the current
- * task.
+ * task *or* the state of the corresponding KVM vcpu if userspace is behaving
+ * as a VMM and that the vcpu has used FP during its last run. In the latter
+ * case, KVM will set TIF_FOREIGN_FPSTATE on kvm_vcpu_put(). For all intents
+ * and purposes, the vcpu FP state is treated identically to userspace's.
  *
  * In order to allow softirq handlers to use FPSIMD, kernel_neon_begin() may
  * save the task's FPSIMD context back to task_struct from softirq context.
-- 
2.30.2

_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

WARNING: multiple messages have this Message-ID (diff)
From: Marc Zyngier <maz@kernel.org>
To: kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Cc: James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Quentin Perret <qperret@google.com>,
	Will Deacon <will@kernel.org>,
	broonie@kernel.org, kernel-team@android.com
Subject: [PATCH 4/4] arm64/fpsimd: Document the use of TIF_FOREIGN_FPSTATE by KVM
Date: Thu, 21 Oct 2021 16:11:24 +0100	[thread overview]
Message-ID: <20211021151124.3098113-5-maz@kernel.org> (raw)
In-Reply-To: <20211021151124.3098113-1-maz@kernel.org>

The bit of documentation that talks about TIF_FOREIGN_FPSTATE
does not mention the ungodly tricks that KVM plays with this flag.

Try and document this for the posterity.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kernel/fpsimd.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index ff4962750b3d..65af2ed64c6a 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -78,7 +78,10 @@
  * indicate whether or not the userland FPSIMD state of the current task is
  * present in the registers. The flag is set unless the FPSIMD registers of this
  * CPU currently contain the most recent userland FPSIMD state of the current
- * task.
+ * task *or* the state of the corresponding KVM vcpu if userspace is behaving
+ * as a VMM and that the vcpu has used FP during its last run. In the latter
+ * case, KVM will set TIF_FOREIGN_FPSTATE on kvm_vcpu_put(). For all intents
+ * and purposes, the vcpu FP state is treated identically to userspace's.
  *
  * In order to allow softirq handlers to use FPSIMD, kernel_neon_begin() may
  * save the task's FPSIMD context back to task_struct from softirq context.
-- 
2.30.2


_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

  parent reply	other threads:[~2021-10-21 15:11 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-21 15:11 [PATCH 0/4] KVM: arm64: Stop mapping current thread_info at EL2 Marc Zyngier
2021-10-21 15:11 ` Marc Zyngier
2021-10-21 15:11 ` Marc Zyngier
2021-10-21 15:11 ` [PATCH 1/4] KVM: arm64: Reorder vcpu flag definitions Marc Zyngier
2021-10-21 15:11   ` Marc Zyngier
2021-10-21 15:11   ` Marc Zyngier
2021-10-21 15:11 ` [PATCH 2/4] KVM: arm64: Introduce flag shadowing TIF_FOREIGN_FPSTATE Marc Zyngier
2021-10-21 15:11   ` Marc Zyngier
2021-10-21 15:11   ` Marc Zyngier
2021-10-21 15:24   ` Mark Brown
2021-10-21 15:24     ` Mark Brown
2021-10-21 15:24     ` Mark Brown
2021-10-21 15:11 ` [PATCH 3/4] KVM: arm64: Stop mapping current thread_info at EL2 Marc Zyngier
2021-10-21 15:11   ` Marc Zyngier
2021-10-21 15:11   ` Marc Zyngier
2021-10-21 15:11 ` Marc Zyngier [this message]
2021-10-21 15:11   ` [PATCH 4/4] arm64/fpsimd: Document the use of TIF_FOREIGN_FPSTATE by KVM Marc Zyngier
2021-10-21 15:11   ` Marc Zyngier
2021-10-21 15:57   ` Mark Brown
2021-10-21 15:57     ` Mark Brown
2021-10-21 15:57     ` Mark Brown
2021-10-27 11:26     ` Marc Zyngier
2021-10-27 11:26       ` Marc Zyngier
2021-10-27 11:26       ` Marc Zyngier

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