linux-kernel - Re: [RFC 5/8] KVM: arm64: Explicitly handle MDSELR

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Message-ID: <864jc1soxx.wl-maz@kernel.org>
Date: Tue, 16 Apr 2024 09:15:54 +0100
From: Marc Zyngier <maz@...nel.org>
To: Anshuman Khandual <anshuman.khandual@....com>
Cc: linux-arm-kernel@...ts.infradead.org,
	Jonathan Corbet <corbet@....net>,
	Oliver Upton <oliver.upton@...ux.dev>,
	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>,
	Mark Rutland <mark.rutland@....com>,
	kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC 5/8] KVM: arm64: Explicitly handle MDSELR_EL1 traps as UNDEFINED

On Tue, 16 Apr 2024 06:46:13 +0100,
Anshuman Khandual <anshuman.khandual@....com> wrote:
> 
> On 4/12/24 16:35, Marc Zyngier wrote:
> > kvm_init_nv_sysregs() to ensure that these new registers have the
> > correct RES0/RES1 behaviour depending on the supported feature set for
> > the guest.
> 
> Following might be sufficient for MDSELR_EL1, but wondering if these fine
> grained control registers (HDFG[RW]TR2_EL2) need to be completely defined
> for the entire guest feature set, probably required.

Yes, you should check for all features defining a valid bit in these
registers, and apply the correct mask if the feature isn't advertised
to the guest, even if KVM doesn't currently support the feature at
all. This is a bit cumbersome at first, but we don't have to revisit
it when the feature gets enabled, which is a massive maintainability
improvement.

It also means that we just have to read the documentation and match it
against the code, which should be pretty trivial.

> 
>        /* HDFG[RW]TR2_EL2 */
>        res0 = res1 = 0;
>        if (!kvm_has_feat(kvm, ID_AA64DFR0_EL1, DebugVer, V8P9))
>                res0 |= HDFGRTR2_EL2_nMDSELR_EL1;
>        set_sysreg_masks(kvm, HDFGRTR2_EL2, res0 | HDFGRTR2_EL2_RES0, res1);
>        set_sysreg_masks(kvm, HDFGWTR2_EL2, res0 | HDFGWTR2_EL2_RES0, res1);

Yup, this looks sensible for that particular bit. A few more to
go... ;-)

Thanks,

	M.

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