Hi Mark,
On 2/4/25 4:20 PM, Mark Rutland wrote:
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:
I tested the above test case with the whole series.
Tested-by: Eric Auger eric.auger@redhat.com
Thanks
Eric
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 Cc: stable@vger.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: Mark Brown broonie@kernel.org Cc: Oliver Upton oliver.upton@linux.dev Cc: Paolo Bonzini pbonzini@redhat.com Cc: Will Deacon will@kernel.org Signed-off-by: Mark Rutland mark.rutland@arm.com
arch/arm64/kernel/fpsimd.c | 25 ------------------------- arch/arm64/kvm/fpsimd.c | 35 ++++++++++------------------------- 2 files changed, 10 insertions(+), 50 deletions(-)
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c index 2b601d88762d4..8370d55f03533 100644 --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -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
diff --git a/arch/arm64/kvm/fpsimd.c b/arch/arm64/kvm/fpsimd.c index 4d3d1a2eb1570..ceeb0a4893aa7 100644 --- a/arch/arm64/kvm/fpsimd.c +++ b/arch/arm64/kvm/fpsimd.c @@ -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(¤t->thread.uw.fpsimd_state);
- *host_data_ptr(fpmr_ptr) = kern_hyp_va(¤t->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();}/*