Re: [PATCH v3 3/6] arm64: KVM: Add iflag for FEAT_TRF

[Marc Zyngier <maz@kernel.org>]

On Thu, 19 Oct 2023 17:55:01 +0100,
James Clark <james.clark@arm.com> wrote:
> 
> Add an extra iflag to signify if the TRFCR register is accessible.
> Because TRBE requires FEAT_TRF, DEBUG_STATE_SAVE_TRBE still has the same
> behavior even though it's only set when FEAT_TRF is present.
> 
> The following holes are left in struct kvm_vcpu_arch, but there aren't
> enough other 8 bit fields to rearrange it to leave any hole smaller than
> 7 bytes:
> 
>   u8                         cflags;               /*  2292     1 */
>   /* XXX 1 byte hole, try to pack */
>   u16                        iflags;               /*  2294     2 */
>   u8                         sflags;               /*  2296     1 */
>   bool                       pause;                /*  2297     1 */
>   /* XXX 6 bytes hole, try to pack */
> 
> Signed-off-by: James Clark <james.clark@arm.com>
> ---
>  arch/arm64/include/asm/kvm_host.h |  4 +++-
>  arch/arm64/kvm/debug.c            | 22 ++++++++++++++++++----
>  2 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 7c82927ddaf2..0f0bf8e641bd 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -535,7 +535,7 @@ struct kvm_vcpu_arch {
>  	u8 cflags;
>  
>  	/* Input flags to the hypervisor code, potentially cleared after use */
> -	u8 iflags;
> +	u16 iflags;
>  
>  	/* State flags for kernel bookkeeping, unused by the hypervisor code */
>  	u8 sflags;
> @@ -741,6 +741,8 @@ struct kvm_vcpu_arch {
>  #define DEBUG_STATE_SAVE_TRBE	__vcpu_single_flag(iflags, BIT(6))
>  /* vcpu running in HYP context */
>  #define VCPU_HYP_CONTEXT	__vcpu_single_flag(iflags, BIT(7))
> +/* Save trace filter controls */
> +#define DEBUG_STATE_SAVE_TRFCR	__vcpu_single_flag(iflags, BIT(8))
>  
>  /* SVE enabled for host EL0 */
>  #define HOST_SVE_ENABLED	__vcpu_single_flag(sflags, BIT(0))
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 8725291cb00a..20cdd40b3c42 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -331,14 +331,28 @@ void kvm_arch_vcpu_load_debug_state_flags(struct kvm_vcpu *vcpu)
>  	    !(read_sysreg_s(SYS_PMBIDR_EL1) & BIT(PMBIDR_EL1_P_SHIFT)))
>  		vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_SPE);
>  
> -	/* Check if we have TRBE implemented and available at the host */
> -	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_TraceBuffer_SHIFT) &&
> -	    !(read_sysreg_s(SYS_TRBIDR_EL1) & TRBIDR_EL1_P))
> -		vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
> +	/*
> +	 * Save TRFCR on nVHE if FEAT_TRF (TraceFilt) exists. This will be
> +	 * done in cases where use of TRBE doesn't completely disable trace and
> +	 * handles the exclude_host/exclude_guest rules of the trace session.

This comment provides zero information. What will be done? Under which
conditions? What are the rules?

> +	 */
> +	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_TraceFilt_SHIFT)) {
> +		vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_TRFCR);
> +		/*
> +		 * Check if we have TRBE implemented and available at the host. If it's
> +		 * in use at the time of guest switch it will need to be disabled and
> +		 * then restored. The architecture mandates FEAT_TRF with TRBE, so we
> +		 * only need to check for TRBE after TRF.
> +		 */
> +		if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_TraceBuffer_SHIFT) &&
> +		    !(read_sysreg_s(SYS_TRBIDR_EL1) & TRBIDR_EL1_P))
> +			vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
> +	}

Multiple questions:

- Why is it safe to trust the local CPU's capability rather than the
  consolidated view from the cpufeature infrastructure?

- Why defer the saving of the registers if there are no changes made
  to them in the interval?

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.