[PATCH 1/8] KVM: arm64: Unconditionally save+flush host FPSIMD/SVE/SME state

[Eric Auger]

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

I tested the above test case with the whole series.

Tested-by: Eric Auger <eric.auger at 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 at redhat.com>
> Reported-by: Wilco Dijkstra <wilco.dijkstra at arm.com>
> Cc: stable at vger.kernel.org
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Florian Weimer <fweimer at redhat.com>
> Cc: Fuad Tabba <tabba at google.com>
> Cc: Jeremy Linton <jeremy.linton at arm.com>
> Cc: Marc Zyngier <maz at kernel.org>
> Cc: Mark Brown <broonie at kernel.org>
> Cc: Oliver Upton <oliver.upton at linux.dev>
> Cc: Paolo Bonzini <pbonzini at redhat.com>
> Cc: Will Deacon <will at kernel.org>
> Signed-off-by: Mark Rutland <mark.rutland at 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(&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();
> -		}
>  	}
>  
>  	/*