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 ]
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 0137d987631e089c935560eccbc678a258580232..bd4f6c6ee0f318e1f60d40755c0f4915edb415b8 100644 (file)
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -1707,31 +1707,6 @@ 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 8c1d0d4853df48abf4d089bbde153bcee8d0e6d0..8b55de502c8c220e15e3a6b782d5012b9349b612 100644 (file)
--- a/arch/arm64/kvm/fpsimd.c
+++ b/arch/arm64/kvm/fpsimd.c
@@ -79,14 +79,16 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
        if (!system_supports_fpsimd())
                return;
 
-       fpsimd_kvm_prepare();
-
        /*
-        * We will check TIF_FOREIGN_FPSTATE just before entering the
-        * guest in kvm_arch_vcpu_ctxflush_fp() and override this to
-        * FP_STATE_FREE if the flag set.
+        * 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}.
         */
-       vcpu->arch.fp_state = FP_STATE_HOST_OWNED;
+       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)
@@ -96,23 +98,6 @@ 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 PSTATE.SM is enabled then save any pending FP
-                * state and disable PSTATE.SM. If we leave PSTATE.SM
-                * enabled and the guest does not enable SME via
-                * CPACR_EL1.SMEN then operations that should be valid
-                * may generate SME traps from EL1 to EL1 which we
-                * can't intercept and which would confuse the guest.
-                *
-                * Do the same for PSTATE.ZA in the case where there
-                * is state in the registers which has not already
-                * been saved, this is very unlikely to happen.
-                */
-               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();
-               }
        }
 }