Re: [V14 5/8] KVM: arm64: nvhe: Disable branch generation in nVHE guests

[Anshuman Khandual]

On 11/23/23 19:24, James Clark wrote:
> 
> 
> On 21/11/2023 11:12, Anshuman Khandual wrote:
>>
>>
>> On 11/14/23 14:46, James Clark wrote:
>>>
>>>
>>> On 14/11/2023 05:13, Anshuman Khandual wrote:
>>>> Disable the BRBE before we enter the guest, saving the status and enable it
>>>> back once we get out of the guest. This is just to avoid capturing records
>>>> in the guest kernel/userspace, which would be confusing the samples.
>>>>
>>>> Cc: Marc Zyngier <maz@xxxxxxxxxx>
>>>> Cc: Oliver Upton <oliver.upton@xxxxxxxxx>
>>>> Cc: James Morse <james.morse@xxxxxxx>
>>>> Cc: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
>>>> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
>>>> Cc: Will Deacon <will@xxxxxxxxxx>
>>>> Cc: kvmarm@xxxxxxxxxxxxxxx
>>>> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
>>>> CC: linux-kernel@xxxxxxxxxxxxxxx
>>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
>>>> ---
>>>> Changes in V14:
>>>>
>>>> - This is a new patch in the series
>>>>
>>>>  arch/arm64/include/asm/kvm_host.h  |  4 ++++
>>>>  arch/arm64/kvm/debug.c             |  6 +++++
>>>>  arch/arm64/kvm/hyp/nvhe/debug-sr.c | 38 ++++++++++++++++++++++++++++++
>>>>  3 files changed, 48 insertions(+)
>>>>
>>>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>>>> index 68421c74283a..1faa0430d8dd 100644
>>>> --- a/arch/arm64/include/asm/kvm_host.h
>>>> +++ b/arch/arm64/include/asm/kvm_host.h
>>>> @@ -449,6 +449,8 @@ enum vcpu_sysreg {
>>>>  	CNTHV_CVAL_EL2,
>>>>  	PMSCR_EL1,	/* Statistical profiling extension */
>>>>  	TRFCR_EL1,	/* Self-hosted trace filters */
>>>> +	BRBCR_EL1,	/* Branch Record Buffer Control Register */
>>>> +	BRBFCR_EL1,	/* Branch Record Buffer Function Control Register */
>>>>  
>>>>  	NR_SYS_REGS	/* Nothing after this line! */
>>>>  };
>>>> @@ -753,6 +755,8 @@ struct kvm_vcpu_arch {
>>>>  #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))
>>>> +/* Save BRBE context if active  */
>>>> +#define DEBUG_STATE_SAVE_BRBE	__vcpu_single_flag(iflags, BIT(9))
>>>>  
>>>>  /* 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 2ab41b954512..4055783c3d34 100644
>>>> --- a/arch/arm64/kvm/debug.c
>>>> +++ b/arch/arm64/kvm/debug.c
>>>> @@ -354,6 +354,11 @@ void kvm_arch_vcpu_load_debug_state_flags(struct kvm_vcpu *vcpu)
>>>>  		    !(read_sysreg_s(SYS_TRBIDR_EL1) & TRBIDR_EL1_P))
>>>>  			vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
>>>>  	}
>>>> +
>>>> +	/* Check if we have BRBE implemented and available at the host */
>>>> +	if (cpuid_feature_extract_unsigned_field(dfr0, ID_AA64DFR0_EL1_BRBE_SHIFT) &&
>>>> +	    (read_sysreg_s(SYS_BRBCR_EL1) & (BRBCR_ELx_E0BRE | BRBCR_ELx_ExBRE)))
>>>> +		vcpu_set_flag(vcpu, DEBUG_STATE_SAVE_BRBE);
>>>
>>> Isn't this supposed to just be the feature check? Whether BRBE is
>>> enabled or not is checked later in __debug_save_brbe() anyway.
>>
>> Okay, will make it just a feature check via ID_AA64DFR0_EL1_BRBE_SHIFT.
>>
>>>
>>> It seems like it's possible to become enabled after this flag load part.
>>
>> Agreed.
>>
>>>
>>>>  }
>>>>  
>>>>  void kvm_arch_vcpu_put_debug_state_flags(struct kvm_vcpu *vcpu)
>>>> @@ -361,6 +366,7 @@ void kvm_arch_vcpu_put_debug_state_flags(struct kvm_vcpu *vcpu)
>>>>  	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_SPE);
>>>>  	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_TRBE);
>>>>  	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_TRFCR);
>>>> +	vcpu_clear_flag(vcpu, DEBUG_STATE_SAVE_BRBE);
>>>>  }
>>>>  
>>>>  void kvm_etm_set_guest_trfcr(u64 trfcr_guest)
>>>> diff --git a/arch/arm64/kvm/hyp/nvhe/debug-sr.c b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
>>>> index 6174f710948e..e44a1f71a0f8 100644
>>>> --- a/arch/arm64/kvm/hyp/nvhe/debug-sr.c
>>>> +++ b/arch/arm64/kvm/hyp/nvhe/debug-sr.c
>>>> @@ -93,6 +93,38 @@ static void __debug_restore_trace(struct kvm_cpu_context *host_ctxt,
>>>>  		write_sysreg_s(ctxt_sys_reg(host_ctxt, TRFCR_EL1), SYS_TRFCR_EL1);
>>>>  }
>>>>  
>>>> +static void __debug_save_brbe(struct kvm_cpu_context *host_ctxt)
>>>> +{
>>>> +	ctxt_sys_reg(host_ctxt, BRBCR_EL1) = 0;
>>>> +	ctxt_sys_reg(host_ctxt, BRBFCR_EL1) = 0;
>>>> +
>>>> +	/* Check if the BRBE is enabled */
>>>> +	if (!(ctxt_sys_reg(host_ctxt, BRBCR_EL1) & (BRBCR_ELx_E0BRE | BRBCR_ELx_ExBRE)))
>>>> +		return;
>>>
>>> Doesn't this always fail, the host BRBCR_EL1 value was just cleared on
>>> the line above.
>>
>> Agreed, this error might have slipped in while converting to ctxt_sys_reg().
>>
>>>
>>> Also, you need to read the register to determine if it was enabled or
>>
>> Right
>>
>>> not, so you might as well always store the real value, rather than 0 in
>>> the not enabled case.
>>
>> But if it is not enabled - why store the real value ?
>>
> 
> It's fewer lines of code and it's less likely to catch someone out if
> it's always set to whatever the host value was. Using 0 as a special
> value could also be an issue because it's indistinguishable from if the
> register was actually set to 0. It's just more to reason about when you
> could reduce it to a single assignment.
> 
> Also it probably would have avoided the current mistake if it was always
> assigned to the host value as well.

Okay, will always save SYS_BRBCR_EL1 into ctxt_sys_reg(host_ctxt, BRBCR_EL1).

>
>>>
>>>> +
>>>> +	/*
>>>> +	 * Prohibit branch record generation while we are in guest.
>>>> +	 * Since access to BRBCR_EL1 and BRBFCR_EL1 is trapped, the
>>>> +	 * guest can't modify the filtering set by the host.
>>>> +	 */
>>>> +	ctxt_sys_reg(host_ctxt, BRBCR_EL1) = read_sysreg_s(SYS_BRBCR_EL1);
>>>> +	ctxt_sys_reg(host_ctxt, BRBFCR_EL1) = read_sysreg_s(SYS_BRBFCR_EL1)
>>>> +	write_sysreg_s(0, SYS_BRBCR_EL1);
>>>> +	write_sysreg_s(0, SYS_BRBFCR_EL1);
>>>
>>> Why does SYS_BRBFCR_EL1 need to be saved and restored? Only
>>> BRBCR_ELx_E0BRE and BRBCR_ELx_ExBRE need to be cleared to disable BRBE.
>>
>> Right, just thought both brbcr, and brbfcr system registers represent
>> current BRBE state (besides branch records), in a more comprehensive
>> manner, although none would be changed from inside the guest.
>>
> 
> The comment above doesn't match up with this explanation.
> 
> Having it in the code implies that it's needed. And as you say the
> branch records are missing anyway, so you can't even infer that it's
> only done to be comprehensive.
> 
> It would be better to not make anyone reading it wonder why it's done
> and just not do it. It's only 8 bytes but it's also a waste of space.

Sure, will drop BRBFCR_EL1 handling in here. The changed code is something
like as follows.

static void __debug_save_brbe(struct kvm_cpu_context *host_ctxt)
{
        ctxt_sys_reg(host_ctxt, BRBCR_EL1) = read_sysreg_s(SYS_BRBCR_EL1);

        /* Check if the BRBE is enabled */
        if (!(ctxt_sys_reg(host_ctxt, BRBCR_EL1) & (BRBCR_ELx_E0BRE | BRBCR_ELx_ExBRE)))
                return;

        /*
         * Prohibit branch record generation while we are in guest.
         * Since access to BRBCR_EL1 is trapped, the guest can't
         * modify the filtering set by the host.
         */
        write_sysreg_s(0, SYS_BRBCR_EL1);
        isb();
}

static void __debug_restore_brbe(struct kvm_cpu_context *host_ctxt)
{
        if (!ctxt_sys_reg(host_ctxt, BRBCR_EL1))
                return;

        /* Restore BRBE controls */
        write_sysreg_s(ctxt_sys_reg(host_ctxt, BRBCR_EL1), SYS_BRBCR_EL1);
        isb();
}