[PATCH v2 3/7] arm64/fpsimd: Have KVM explicitly say which FP registers to save

[Mark Brown <broonie@kernel.org>]

In order to avoid needlessly saving and restoring the guest registers KVM
relies on the host FPSMID code to save the guest registers when we context
switch away from the guest. This is done by binding the KVM guest state to
the CPU on top of the task state that was originally there, then carefully
managing the TIF_SVE flag for the task to cause the host to save the full
SVE state when needed regardless of the needs of the host task. This works
well enough but isn't terribly direct about what is going on and makes it
much more complicated to try to optimise what we're doing with the SVE
register state.

Let's instead have KVM pass in the register state it wants saving when it
binds to the CPU. We introduce a new FP_TYPE_TASK for use during normal
task binding to indicate that we should base our decisions on the current
task. In order to ease any future debugging that might be required this
patch does not actually update any of the decision making about what to
save, it merely starts tracking the new information and warns if the
requested state is not what we would otherwise have decided to save.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 arch/arm64/include/asm/fpsimd.h    |  3 ++-
 arch/arm64/include/asm/processor.h |  1 +
 arch/arm64/kernel/fpsimd.c         | 20 +++++++++++++++++++-
 arch/arm64/kvm/fpsimd.c            |  9 ++++++++-
 4 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
index 5762419fdcc0..e008965719a4 100644
--- a/arch/arm64/include/asm/fpsimd.h
+++ b/arch/arm64/include/asm/fpsimd.h
@@ -61,7 +61,8 @@ extern void fpsimd_kvm_prepare(void);
 extern void fpsimd_bind_state_to_cpu(struct user_fpsimd_state *state,
 				     void *sve_state, unsigned int sve_vl,
 				     void *za_state, unsigned int sme_vl,
-				     u64 *svcr, enum fp_state *type);
+				     u64 *svcr, enum fp_state *type,
+				     enum fp_state to_save);
 
 extern void fpsimd_flush_task_state(struct task_struct *target);
 extern void fpsimd_save_and_flush_cpu_state(void);
diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
index 192986509a8e..7d9f0c95b352 100644
--- a/arch/arm64/include/asm/processor.h
+++ b/arch/arm64/include/asm/processor.h
@@ -123,6 +123,7 @@ enum vec_type {
 };
 
 enum fp_state {
+	FP_STATE_TASK,		/* Save based on current, invalid as fp_type */
 	FP_STATE_FPSIMD,
 	FP_STATE_SVE,
 };
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index fdb2925becdf..95c95411bd42 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -126,6 +126,7 @@ struct fpsimd_last_state_struct {
 	unsigned int sve_vl;
 	unsigned int sme_vl;
 	enum fp_state *type;
+	enum fp_state to_save;
 };
 
 static DEFINE_PER_CPU(struct fpsimd_last_state_struct, fpsimd_last_state);
@@ -458,6 +459,21 @@ static void fpsimd_save(void)
 		vl = last->sve_vl;
 	}
 
+	/*
+	 * For now we're just validating that the requested state is
+	 * consistent with what we'd otherwise work out.
+	 */
+	switch (last->to_save) {
+	case FP_STATE_TASK:
+		break;
+	case FP_STATE_FPSIMD:
+		WARN_ON_ONCE(save_sve_regs);
+		break;
+	case FP_STATE_SVE:
+		WARN_ON_ONCE(!save_sve_regs);
+		break;
+	}
+
 	if (system_supports_sme()) {
 		u64 *svcr = last->svcr;
 		*svcr = read_sysreg_s(SYS_SVCR);
@@ -1702,6 +1718,7 @@ static void fpsimd_bind_task_to_cpu(void)
 	last->sme_vl = task_get_sme_vl(current);
 	last->svcr = &current->thread.svcr;
 	last->type = &current->thread.fp_type;
+	last->to_save = FP_STATE_TASK;
 	current->thread.fpsimd_cpu = smp_processor_id();
 
 	/*
@@ -1726,7 +1743,7 @@ static void fpsimd_bind_task_to_cpu(void)
 void fpsimd_bind_state_to_cpu(struct user_fpsimd_state *st, void *sve_state,
 			      unsigned int sve_vl, void *za_state,
 			      unsigned int sme_vl, u64 *svcr,
-			      enum fp_state *type)
+			      enum fp_state *type, enum fp_state to_save)
 {
 	struct fpsimd_last_state_struct *last =
 		this_cpu_ptr(&fpsimd_last_state);
@@ -1741,6 +1758,7 @@ void fpsimd_bind_state_to_cpu(struct user_fpsimd_state *st, void *sve_state,
 	last->sve_vl = sve_vl;
 	last->sme_vl = sme_vl;
 	last->type = type;
+	last->to_save = to_save;
 }
 
 /*
diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index be3ddb214ab1..542c71b16451 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -129,9 +129,16 @@ void kvm_arch_vcpu_ctxflush_fp(struct kvm_vcpu *vcpu)
  */
 void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu)
 {
+	enum fp_state fp_type;
+
 	WARN_ON_ONCE(!irqs_disabled());
 
 	if (vcpu->arch.flags & KVM_ARM64_FP_ENABLED) {
+		if (vcpu_has_sve(vcpu))
+			fp_type = FP_STATE_SVE;
+		else
+			fp_type = FP_STATE_FPSIMD;
+
 		/*
 		 * Currently we do not support SME guests so SVCR is
 		 * always 0 and we just need a variable to point to.
@@ -140,7 +147,7 @@ void kvm_arch_vcpu_ctxsync_fp(struct kvm_vcpu *vcpu)
 					 vcpu->arch.sve_state,
 					 vcpu->arch.sve_max_vl,
 					 NULL, 0, &vcpu->arch.svcr,
-					 &vcpu->arch.fp_type);
+					 &vcpu->arch.fp_type, fp_type);
 
 		clear_thread_flag(TIF_FOREIGN_FPSTATE);
 		update_thread_flag(TIF_SVE, vcpu_has_sve(vcpu));
-- 
2.30.2

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