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>

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index e8f10daaa0d7a21540157c525120a49feea72f37..4be9d9fd4fb716cb1ad6a8e527e5fe81255d12c5 100644 (file)
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1154,31 +1154,6 @@ void fpsimd_signal_preserve_current_state(void)
                sve_to_fpsimd(current);
 }
 
-/*
- * 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

diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c
index 2e0f44f4c470a7fc89ddcce708b4261150e8626f..2afa2521bce133153abcd2a322d365a018f35a15 100644 (file)
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -74,9 +74,16 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
        vcpu->arch.flags &= ~KVM_ARM64_FP_ENABLED;
        vcpu->arch.flags |= KVM_ARM64_FP_HOST;
 
-       fpsimd_kvm_prepare();
-
-       vcpu->arch.flags &= ~KVM_ARM64_HOST_SVE_ENABLED;
+       /*
+        * 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.flags &= ~KVM_ARM64_FP_HOST;
 
        if (read_sysreg(cpacr_el1) & CPACR_EL1_ZEN_EL0EN)
                vcpu->arch.flags |= KVM_ARM64_HOST_SVE_ENABLED;