KVM: arm64: Unconditionally save+flush host FPSIMD/SVE/SME state

[ Upstream commit fbc7e61195e23f744814e78524b73b59faa54ab4 ]

There are several problems with the way hyp code lazily saves the host's
FPSIMD/SVE state, including:

* Host SVE being discarded unexpectedly due to inconsistent
  configuration of TIF_SVE and CPACR_ELx.ZEN. This has been seen to
  result in QEMU crashes where SVE is used by memmove(), as reported by
  Eric Auger:

  https://issues.redhat.com/browse/RHEL-68997

* Host SVE state is discarded *after* modification by ptrace, which was an
  unintentional ptrace ABI change introduced with lazy discarding of SVE state.

* The host FPMR value can be discarded when running a non-protected VM,
  where FPMR support is not exposed to a VM, and that VM uses
  FPSIMD/SVE. In these cases the hyp code does not save the host's FPMR
  before unbinding the host's FPSIMD/SVE/SME state, leaving a stale
  value in memory.

Avoid these by eagerly saving and "flushing" the host's FPSIMD/SVE/SME
state when loading a vCPU such that KVM does not need to save any of the
host's FPSIMD/SVE/SME state. For clarity, fpsimd_kvm_prepare() is
removed and the necessary call to fpsimd_save_and_flush_cpu_state() is
placed in kvm_arch_vcpu_load_fp(). As 'fpsimd_state' and 'fpmr_ptr'
should not be used, they are set to NULL; all uses of these will be
removed in subsequent patches.

Historical problems go back at least as far as v5.17, e.g. erroneous
assumptions about TIF_SVE being clear in commit:

  8383741ab2e773a9 ("KVM: arm64: Get rid of host SVE tracking/saving")

... and so this eager save+flush probably needs to be backported to ALL
stable trees.

Fixes: 93ae6b01bafee8fa ("KVM: arm64: Discard any SVE state when entering KVM guests")
Fixes: 8c845e2731041f0f ("arm64/sve: Leave SVE enabled on syscall if we don't context switch")
Fixes: ef3be86021c3bdf3 ("KVM: arm64: Add save/restore support for FPMR")
Reported-by: Eric Auger <eauger@redhat.com>
Reported-by: Wilco Dijkstra <wilco.dijkstra@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Tested-by: Mark Brown <broonie@kernel.org>
Tested-by: Eric Auger <eric.auger@redhat.com>
Acked-by: Will Deacon <will@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Fuad Tabba <tabba@google.com>
Cc: Jeremy Linton <jeremy.linton@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250210195226.1215254-2-mark.rutland@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>
[ Mark: Handle vcpu/host flag conflict, remove host_data_ptr() ]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 10 insertions, 33 deletions

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 3fcacbce5d42..47425311acc5 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1672,31 +1672,6 @@ void fpsimd_signal_preserve_current_state(void)
 }
 
 /*
- * Called by KVM when entering the guest.
- */
-void fpsimd_kvm_prepare(void)
-{
-	if (!system_supports_sve())
-		return;
-
-	/*
-	 * KVM does not save host SVE state since we can only enter
-	 * the guest from a syscall so the ABI means that only the
-	 * non-saved SVE state needs to be saved.  If we have left
-	 * SVE enabled for performance reasons then update the task
-	 * state to be FPSIMD only.
-	 */
-	get_cpu_fpsimd_context();
-
-	if (test_and_clear_thread_flag(TIF_SVE)) {
-		sve_to_fpsimd(current);
-		current->thread.fp_type = FP_STATE_FPSIMD;
-	}
-
-	put_cpu_fpsimd_context();
-}
-
-/*
  * Associate current's FPSIMD context with this cpu
  * The caller must have ownership of the cpu FPSIMD context before calling
  * this function.
diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index ec82d0191f76..1765f723afd4 100644
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -79,9 +79,16 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
 	if (!system_supports_fpsimd())
 		return;
 
-	fpsimd_kvm_prepare();
-
-	vcpu->arch.fp_state = FP_STATE_HOST_OWNED;
+	/*
+	 * Ensure that any host FPSIMD/SVE/SME state is saved and unbound such
+	 * that the host kernel is responsible for restoring this state upon
+	 * return to userspace, and the hyp code doesn't need to save anything.
+	 *
+	 * When the host may use SME, fpsimd_save_and_flush_cpu_state() ensures
+	 * that PSTATE.{SM,ZA} == {0,0}.
+	 */
+	fpsimd_save_and_flush_cpu_state();
+	vcpu->arch.fp_state = FP_STATE_FREE;
 
 	vcpu_clear_flag(vcpu, HOST_SVE_ENABLED);
 	if (read_sysreg(cpacr_el1) & CPACR_EL1_ZEN_EL0EN)
@@ -100,11 +107,6 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
 		vcpu_clear_flag(vcpu, HOST_SME_ENABLED);
 		if (read_sysreg(cpacr_el1) & CPACR_EL1_SMEN_EL0EN)
 			vcpu_set_flag(vcpu, HOST_SME_ENABLED);
-
-		if (read_sysreg_s(SYS_SVCR) & (SVCR_SM_MASK | SVCR_ZA_MASK)) {
-			vcpu->arch.fp_state = FP_STATE_FREE;
-			fpsimd_save_and_flush_cpu_state();
-		}
 	}
 }