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

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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:

  8383741ab2 ("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: 93ae6b01ba ("KVM: arm64: Discard any SVE state when entering KVM guests")
Fixes: 8c845e2731 ("arm64/sve: Leave SVE enabled on syscall if we don't context switch")
Fixes: ef3be86021 ("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>
This commit is contained in:
Mark Rutland 2025-02-10 19:52:19 +00:00 • committed by Marc Zyngier
parent 8dbccafce3
commit fbc7e61195
2 changed files with 10 additions and 50 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

25
arch/arm64/kernel/fpsimd.c
View file

@ -1694,31 +1694,6 @@ 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

35
arch/arm64/kvm/fpsimd.c
View file

@ -54,16 +54,18 @@ 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}.
*/
*host_data_ptr(fp_owner) = FP_STATE_HOST_OWNED;
*host_data_ptr(fpsimd_state) = kern_hyp_va(&current->thread.uw.fpsimd_state);
*host_data_ptr(fpmr_ptr) = kern_hyp_va(&current->thread.uw.fpmr);
fpsimd_save_and_flush_cpu_state();
*host_data_ptr(fp_owner) = FP_STATE_FREE;
*host_data_ptr(fpsimd_state) = NULL;
*host_data_ptr(fpmr_ptr) = NULL;
host_data_clear_flag(HOST_SVE_ENABLED);
if (read_sysreg(cpacr_el1) & CPACR_EL1_ZEN_EL0EN)
@ -73,23 +75,6 @@ void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu)
host_data_clear_flag(HOST_SME_ENABLED);
if (read_sysreg(cpacr_el1) & CPACR_EL1_SMEN_EL0EN)
host_data_set_flag(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)) {
*host_data_ptr(fp_owner) = FP_STATE_FREE;
fpsimd_save_and_flush_cpu_state();
}
}
/*