Re: [PATCH v1 2/2] KVM: arm64: nv: update CPU register PAR_EL1 after 'at s*'

[Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>]

Hi Marc,

Marc Zyngier <maz@kernel.org> writes:

> On Wed, 06 Aug 2025 15:17:55 +0100,
> Volodymyr Babchuk <Volodymyr_Babchuk@epam.com> wrote:
>> 
>> Previously this code update only vCPU's in-memory value, which is good,
>> but not enough, as there will be no context switch after exiting
>> exception handler, so in-memory value will not get into actual
>> register.
>> 
>> It worked good enough for VHE guests because KVM code tried fast path,
>> which of course updated real PAR_EL1.
>
> Nothing to do with VHE, I'm afraid. We can take the slow path for any
> odd reason, even on VHE.

Yeah, I just didn't conveyed my idea well. As VHE guest does not require
S2 translation, calling real "at s1*" was enough to get correct value in
PAR_EL1 in that case. That would hide that issue if using VHE
guests. Also, I believe that most hypervisors will try to do own
software pagewalk if "at s1" fails...

Anyways, I'll rewrite commit message to make this more clear.

> This is more of a structural problem, see
> below.
>
>> 
>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
>> ---
>>  arch/arm64/kvm/sys_regs.c | 6 ++++++
>>  1 file changed, 6 insertions(+)
>> 
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index 7b8a0a6f26468..ab2b5e261d312 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -3463,6 +3463,9 @@ static bool handle_at_s1e2(struct kvm_vcpu *vcpu, struct sys_reg_params *p,
>>  
>>  	__kvm_at_s1e2(vcpu, op, p->regval);
>>  
>> +	/* No context switch happened, so we need to update PAR_EL1 manually */
>> +	write_sysreg(vcpu_read_sys_reg(vcpu, PAR_EL1), par_el1);
>> +
>
> This looks like the wrong fix, as it papers over another issue.
>
> The core problem is vcpu_write_sys_reg() (resp. read) does the wrong
> thing with registers such as PAR_EL1, which are not translated between
> EL1 and EL2, and therefore are always live, no matter what.
>
> Can you please try the hack below? I don't like it, but at least it
> shows where the actual problem lies.

It does no work, see below.

Also, what do you think about Oliver's approach? I'll try it next.

>
> Thanks,
>
> 	M.
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index ad25484772574..167f0d411a708 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -95,7 +95,13 @@ static bool write_to_read_only(struct kvm_vcpu *vcpu,
>  		return true;						\
>  	}
>  
> -static bool get_el2_to_el1_mapping(unsigned int reg,
> +#define COMMON_SYSREG(r)						\
> +	case r: {							\
> +		 *el1r = __INVALID_SYSREG__;
> \
This didn't worked right away, as code in vcpu_read_sys_reg()
will do just __vcpu_read_sys_reg_from_cpu(el1r, &val);

(the same true for write counterpart of course)

I tried to put  *el1r = r, here, but this didn't worked also, because of
this check:

		/*
		 * If this register does not have an EL1 counterpart,
		 * then read the stored EL2 version.
		 */
		if (reg == el1r)
			goto memory_read;

So, either we need to add one more check, like

if (el1r == INVALID_REG)
    __vcpu_read_sys_reg_from_cpu(reg, &val);

Or use Oliver's approach.


> +		 return is_hyp_ctxt(vcpu);					\
> +	}
> +
> +static bool get_el2_to_el1_mapping(const struct kvm_vcpu *vcpu, unsigned int reg,
>  				   unsigned int *el1r, u64 (**xlate)(u64))
>  {
>  	switch (reg) {
> @@ -119,6 +125,7 @@ static bool get_el2_to_el1_mapping(unsigned int reg,
>  		PURE_EL2_SYSREG(  HAFGRTR_EL2	);
>  		PURE_EL2_SYSREG(  CNTVOFF_EL2	);
>  		PURE_EL2_SYSREG(  CNTHCTL_EL2	);
> +		COMMON_SYSREG(	  PAR_EL1	);
>  		MAPPED_EL2_SYSREG(SCTLR_EL2,   SCTLR_EL1,
>  				  translate_sctlr_el2_to_sctlr_el1	     );
>  		MAPPED_EL2_SYSREG(CPTR_EL2,    CPACR_EL1,
> @@ -158,7 +165,7 @@ u64 vcpu_read_sys_reg(const struct kvm_vcpu *vcpu, int reg)
>  	if (!vcpu_get_flag(vcpu, SYSREGS_ON_CPU))
>  		goto memory_read;
>  
> -	if (unlikely(get_el2_to_el1_mapping(reg, &el1r, &xlate))) {
> +	if (unlikely(get_el2_to_el1_mapping(vcpu, reg, &el1r, &xlate))) {
>  		if (!is_hyp_ctxt(vcpu))
>  			goto memory_read;
>  
> @@ -219,7 +226,7 @@ void vcpu_write_sys_reg(struct kvm_vcpu *vcpu, u64 val, int reg)
>  	if (!vcpu_get_flag(vcpu, SYSREGS_ON_CPU))
>  		goto memory_write;
>  
> -	if (unlikely(get_el2_to_el1_mapping(reg, &el1r, &xlate))) {
> +	if (unlikely(get_el2_to_el1_mapping(vcpu, reg, &el1r, &xlate))) {
>  		if (!is_hyp_ctxt(vcpu))
>  			goto memory_write;

-- 
WBR, Volodymyr