linux-kernel - Re: [PATCH 47/47] KVM: arm64: nv: Add trap forwarding for FEAT

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Message-ID: <Zv92nsoLxbfI45Ji@linux.dev>
Date: Thu, 3 Oct 2024 22:01:18 -0700
From: Oliver Upton <oliver.upton@...ux.dev>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, maz@...nel.org,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH 47/47] KVM: arm64: nv: Add trap forwarding for FEAT_FGT2
 described registers

On Thu, Oct 03, 2024 at 09:46:08AM +0530, Anshuman Khandual wrote:
> > I have a patch in the nested PMU series that uses a single complex trap
> > ID to evaluate HPMN, and derives the index from ESR_EL2. I think it
> > could also be extended to the PMEVCNTSVR<n> range as well.
> 
> Just for reference - the mentioned complex trap ID function from the
> given link below.
> 
> static enum trap_behaviour check_mdcr_hpmn(struct kvm_vcpu *vcpu)
> {
> 	u32 sysreg = esr_sys64_to_sysreg(kvm_vcpu_get_esr(vcpu));
> 	u64 mask = kvm_pmu_accessible_counter_mask(vcpu);
> 	unsigned int idx;
> 
> 
> 	switch (sysreg) {
> 	case SYS_PMEVTYPERn_EL0(0) ... SYS_PMEVTYPERn_EL0(30):
> 	case SYS_PMEVCNTRn_EL0(0) ... SYS_PMEVCNTRn_EL0(30):
> 
> ---------------------------------------------------------------------
> Just add the new system register range here ?
> 
> +	case SYS_PMEVCNTSVR_EL1(0)... SYS_PMEVCNTSVR_EL1(31):
> ---------------------------------------------------------------------
> 
> 		idx = (sys_reg_CRm(sysreg) & 0x3) << 3 | sys_reg_Op2(sysreg);
> 		break;

Yes, so long as the layout of encodings matches the established pattern
for value / type registers (I haven't checked this).

> > 
> > Also, keep in mind that the HPMN trap is annoying since it affects Host
> > EL0 in addition to 'guest' ELs.
> 
> Does this require any more special handling other than the above complex trap
> ID function ?

There's another patch in that series I linked that allows EL2 traps to
describe behavior that takes effect in host EL0.

So I don't believe there's anything in particular related to HPMN that
you need to evaluate. I wanted to mention it because some of the PMU
related traps besides HPMN take effect in Host EL0, so do keep it in
mind.

With that said, I haven't seen an FGT yet that applies to Host EL0.

-- 
Thanks,
Oliver