[PATCH RFC] KVM: arm64: allow ID_MMFR4_EL1 to be writable

[Russell King (Oracle)]

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 think some of these differences are due to Marc's removal of the
WA2 code in commit 29e8910a566a ("KVM: arm64: Simplify handling of
ARCH_WORKAROUND_2"). The WA2 register for example has changed from
avail/enabled to not_avail.

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