Re: [PATCH v2 11/13] KVM: arm64: nv: Add emulation for ERETAx instructions

From: Marc Zyngier <maz@kernel.org>
To: Joey Gouly <joey.gouly@arm.com>
Cc: kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Oliver Upton <oliver.upton@linux.dev>,
	Zenghui Yu <yuzenghui@huawei.com>, Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v2 11/13] KVM: arm64: nv: Add emulation for ERETAx instructions
Date: Fri, 08 Mar 2024 17:54:20 +0000	[thread overview]
Message-ID: <86h6hg1uer.wl-maz@kernel.org> (raw)
In-Reply-To: <20240308172059.GA1052268@e124191.cambridge.arm.com>

On Fri, 08 Mar 2024 17:20:59 +0000,
Joey Gouly <joey.gouly@arm.com> wrote:
> 
> Phew..

[...]

> Each function in this file is quite small, but there's certainly a lot of
> complexity and background knowledge required to understand them!
> 
> I spent quite some time on each part to see if it matches what I understood
> from the Arm ARM.
> 
> Reviewed-by: Joey Gouly <joey.gouly@arm.com>

Thanks a lot for putting up with it, much appreciated.

> A side note / thing I considered. KVM doesn't currently handle ERET exceptions
> from EL1.

EL1 is ambiguous here. Is that EL1 from the PoV of the guest?

>
> 1. If an ERETA{A,B} were executed from a nested EL1 guest, that would be
> trapped up to Host KVM at EL2.

There are two possibilities for that (assuming EL1 from the PoV of a
L1 guest):

(1) this EL1 guest is itself a guest hypervisor (i.e. we are running
    an L1 guest which itself is using NV and running an L2 which
    itself is a hypervisor). In that case, ERET* would have to be
    trapped to EL2 and re-injected. Note that we do not support NV
    under NV. Yet...

(2) the L2 guest is not a hypervisor (no recursive NV), but the L1
    hypervisor has set HFGITR_EL2.ERET==1. We'd have to re-inject the
    exception into L1, just like in the precedent case.

If neither HCR_EL2.NV nor HFGITR_EL2.ERET are set, then no ERET* gets
trapped at all. Crucially, when running an L2 guest that doesn't isn't
itself a hypervisor (no nested NV), we do not trap ERET* at all.

In a way, the NV overhead is mostly when running L1. Once you run L2,
the overhead "vanishes", to some extent (as long as you don't exit,
because that's where the cost is).

> 2. kvm_hyp_handle_eret() returns false since it's not from vEL2.  Inside
> kvm_handle_eret(), is_hyp_ctxt() is false so the exception is injected into
> vEL2 (via kvm_inject_nested_sync()).
> 
> 3. vEL2 gets the exception, kvm_hyp_handle_eret() returns false as before.
> Inside kvm_handle_eret(), is_hyp_ctxt() is also false, so
> kvm_inject_nested_sync() is called but now errors out since vcpu_has_nv() is
> false.
> 
> Is that flow right? Am I missing something?

I'm not sure. The cases where ERET gets trapped are really limited to
the above two cases.

Thanks,

	M.

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