Re: [PATCH v6 11/25] KVM: arm64: Add remaining ID registers to id_reg_desc_table

From: Oliver Upton <oupton@google.com>
To: Ricardo Koller <ricarkol@google.com>
Cc: Reiji Watanabe <reijiw@google.com>, Marc Zyngier <maz@kernel.org>,
	kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <alexandru.elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Will Deacon <will@kernel.org>, Andrew Jones <drjones@redhat.com>,
	Fuad Tabba <tabba@google.com>,
	Peng Liang <liangpeng10@huawei.com>,
	Peter Shier <pshier@google.com>,
	Jing Zhang <jingzhangos@google.com>,
	Raghavendra Rao Anata <rananta@google.com>
Subject: Re: [PATCH v6 11/25] KVM: arm64: Add remaining ID registers to id_reg_desc_table
Date: Wed, 23 Mar 2022 22:25:35 +0000	[thread overview]
Message-ID: <YjueX2DOxjoc/d4j@google.com> (raw)
In-Reply-To: <Yjuds73S1sO1UpJI@google.com>

On Wed, Mar 23, 2022 at 03:22:43PM -0700, Ricardo Koller wrote:
> On Wed, Mar 23, 2022 at 08:44:26PM +0000, Oliver Upton wrote:
> > On Wed, Mar 23, 2022 at 01:13:32PM -0700, Ricardo Koller wrote:
> > > On Wed, Mar 23, 2022 at 07:53:14PM +0000, Oliver Upton wrote:
> > > > Hi Reiji,
> > > > 
> > > > On Thu, Mar 10, 2022 at 08:47:57PM -0800, Reiji Watanabe wrote:
> > > > > Add hidden or reserved ID registers, and remaining ID registers,
> > > > > which don't require special handling, to id_reg_desc_table.
> > > > > Add 'flags' field to id_reg_desc, which is used to indicates hiddden
> > > > > or reserved registers. Since now id_reg_desc_init() is called even
> > > > > for hidden/reserved registers, change it to not do anything for them.
> > > > > 
> > > > > Signed-off-by: Reiji Watanabe <reijiw@google.com>
> > > > 
> > > > I think there is a very important detail of the series that probably
> > > > should be highlighted. We are only allowing AArch64 feature registers to
> > > > be configurable, right? AArch32 feature registers remain visible with
> > > > their default values passed through to the guest. If you've already
> > > > stated this as a precondition elsewhere then my apologies for the noise.
> > > 
> > > Aren't AArch64 ID regs architecturally mapped to their AArch32
> > > counterparts?  They should show the same values.  I'm not sure if it's a
> > > problem (and if KVM is faithful to that rule),
> > 
> > I believe it's a bit more subtle than that. The AArch32 feature registers
> > are architecturally mapped to certain encodings accessible from AArch64.
> > For example, ID_PFR0_EL1 is actually a 64 bit register where bits [31:0]
> > map to the ID_PFR0 AArch32 register. ID_PFR0_EL1 is only accessible from
> > AArch64 with the MRS instruction, and ID_PFR0 is only accessible from
> > AArch32 with the MRC instruction. KVM just so happens to handle both of
> > these reads from the same sys_reg_desc.
> > 
> > AFAIK, there does not exist a direct bit mapping between the
> > ID_*_EL1 <-> ID_AA64*_EL1 registers. But hey, could be wrong :)
> 
> I think you are right. ID_PFR0_EL1[31:0] doesn't even have the same
> field as ID_AA64PFR0_EL1[31:0]. The only exception would be RAS which is
> at [31:28] on both, but it doesn't say anywhere that ID_PFR0_EL1.RAS
> maps architecturally to ID_AA64PFR0_EL1.RAS. So, I think we can assume
> it doesn't (?).

Right, the feature registers are generally related (you will find fields
of similar meaning), but figuring out that tangle and making it work is
going to be a massive waste of time IMO. If we can say that our new
feature configuration is AArch64-only, all potential bugs relating to
AArch32 collapse :-)

--
Thanks,
Oliver

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