From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>, James Morse <james.morse@arm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Zenghui Yu <yuzenghui@huawei.com>,
linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev
Subject: Re: [PATCH RFC] KVM: arm64: allow ID_MMFR4_EL1 to be writable
Date: Tue, 7 May 2024 10:27:23 +0100 [thread overview]
Message-ID: <Zjnz+2Yr/7BLhoG4@shell.armlinux.org.uk> (raw)
In-Reply-To: <865xvwqlup.wl-maz@kernel.org>
On Thu, May 02, 2024 at 04:23:10PM +0100, Marc Zyngier wrote:
> On Thu, 02 May 2024 11:50:10 +0100,
> "Russell King (Oracle)" <linux@armlinux.org.uk> wrote:
> >
> > On Wed, May 01, 2024 at 08:51:15PM +0100, Russell King (Oracle) wrote:
> > > On Wed, May 01, 2024 at 06:59:17PM +0000, Oliver Upton wrote:
> > > > On Wed, May 01, 2024 at 07:08:05PM +0100, Russell King (Oracle) wrote:
> > > > > On Wed, May 01, 2024 at 05:57:20PM +0000, Oliver Upton wrote:
> > > > > > Hi Russell,
> > > > > >
> > > > > > On Wed, May 01, 2024 at 06:06:51PM +0100, Russell King (Oracle) wrote:
> > > > > > > Between 5.4 and 5.15, the guests view of HPDS, CnP, XNX and AC2
> > > > > > > changed their value on the same Neoverse N1 r3p1 hardware which makes
> > > > > > > migrating between these kernels on the host problematical.
> > > > > >
> > > > > > It'd be helpful to expand a bit more on how these fields changed, better
> > > > > > yet if we can blame it back to a commit. I'm guessing the only direction
> > > > > > of migration you care about is old -> new then?
> > > > >
> > > > > Yes. For MMFR4_EL1, we see 0 with our 5.4 based kernel, and 0x21110
> > > > > with our 5.15 kernel. I've been looking at tracking down which commit
> > > > > is responsible but I've come up with nothing that fits.
> > > > >
> > > > > The only change I can see is the FTR definition for MMFR4, but this
> > > > > always included 4:7 (AC2) which changed 0 -> 1. So... no idea what
> > > > > commit caused the change.
> > > > >
> > > > > There are a load of other registers that we need sorting, but this
> > > > > is just a test forray into attempting to solve this.
> > > >
> > > > Got it, let me see if I can find it then. Do share that list of
> > > > problematic registers when you have it, hopefully this isn't the tip of
> > > > the iceberg...
> > >
> > > There unfortunately is an iceberg, but hopefully it isn't big enough to
> > > sink a ship!
> > >
> > > Besides ID_MMFR4_EL1, here are the other differences we've identified.
> > > Note that these are Oracle's UEK kernels, so based on stable kernel
> > > branches.
> > >
> > > Register Field 5.4.x 5.15.x
> > > ID_PFR0_EL1 CSV2 0 1
> > > ID_ISAR6_EL1 DP 0 1
> > > ID_PFR2_EL1 SSBS 0 1
> > > CSV3 0 1
> > > ID_AA64DFR0_EL1 PMSVer 1 0
> > > DebugVer 8 6
> > > ID_AA64MMFR1_EL1 XNX 0 1
> > > ID_AA64MMFR2_EL1 EVT 0 1
> > > KVM_REG_ARM_SMCCC_ARCH_WORKAROUND_2
> > > 0x12 0
> >
> > I'm finding sys_regs.c very unintuitive for working out what we allow
> > to be written, because it's all coded in negative-logic. By that I mean
> > the mask values are all ~(what-we-don't-allow) rather than a positive
> > this-is-what-we-allow. So I've ended up creating a table, looking up
> > the registers and working out what's read-only and what's read-write.
>
> [...]
>
> Using positive or negative logic doesn't really have any impact on the
> result. It often is a matter of concisely expressing what is allowed
> or not.
>
> There is also the fact that a lot of the KVM code now checks for
> "feature downgrade" and enforces it. Construct such as:
>
> if (!kvm_has_feat(kvm, ID_AA64ISAR0_EL1, TLB, OS))
> kvm->arch.fgu[HFGITR_GROUP] |= (HFGITR_EL2_TLBIRVAALE1OS|
> HFGITR_EL2_TLBIRVALE1OS |
> HFGITR_EL2_TLBIRVAAE1OS |
> HFGITR_EL2_TLBIRVAE1OS |
> HFGITR_EL2_TLBIVAALE1OS |
> HFGITR_EL2_TLBIVALE1OS |
> HFGITR_EL2_TLBIVAAE1OS |
> HFGITR_EL2_TLBIASIDE1OS |
> HFGITR_EL2_TLBIVAE1OS |
> HFGITR_EL2_TLBIVMALLE1OS);
>
> use negative logic by expressing what we don't want to enable.
>
> In the end, consistency matters.
Is that a request to change my patch?
I'm unclear whether anyone wants changes to it.
--
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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next prev parent reply other threads:[~2024-05-07 9:30 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-01 17:06 [PATCH RFC] KVM: arm64: allow ID_MMFR4_EL1 to be writable Russell King (Oracle)
2024-05-01 17:57 ` Oliver Upton
2024-05-01 18:08 ` Russell King (Oracle)
2024-05-01 18:59 ` Oliver Upton
2024-05-01 19:51 ` Russell King (Oracle)
2024-05-02 10:50 ` Russell King (Oracle)
2024-05-02 15:23 ` Marc Zyngier
2024-05-07 9:27 ` Russell King (Oracle) [this message]
2024-05-02 14:40 ` Russell King (Oracle)
2024-05-02 16:45 ` Oliver Upton
2024-05-08 12:06 ` Cornelia Huck
2024-05-08 17:14 ` Oliver Upton
2024-05-10 15:11 ` Cornelia Huck
2024-05-13 21:26 ` Oliver Upton
2024-05-22 10:14 ` Cornelia Huck
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