Re: KVM NV + SVE host OS warning

[Marc Zyngier <maz@kernel.org>]

On Wed, 08 Oct 2025 14:43:29 +0100,
Jan Kotas <jank@cadence.com> wrote:
> >>
> >> Is it possible to trap/emulate HCR_EL2, and make sure E2H is always 1,
> >> in cases when only KVM_ARM_VCPU_HAS_EL2 is set?
> >> Would that help?
> > 
> > There are no traps for HCR_EL2.
> > 
> >> As a test, I tried forcing this bit before running VCPUs, but
> >> reading it via MRS got a value without it being set.
> > 
> > Where did you read that from?
> 
> I checked the X1 register, after: mrs x1, hcr_el2 in __check_hvhe.

That's too late. At this point, the write to HCR_EL2 will have already
occurred,

> 
> 
> > If you apply the following change to your guest, does it start
> > behaving?
> > 
> > M.
> > 
> > diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h
> > index 46033027510cc..392c9f4016f2e 100644
> > --- a/arch/arm64/include/asm/el2_setup.h
> > +++ b/arch/arm64/include/asm/el2_setup.h
> > @@ -34,7 +34,7 @@
> > mrs_s x1, SYS_ID_AA64MMFR4_EL1
> > sbfx x1, x1, #ID_AA64MMFR4_EL1_E2H0_SHIFT, #ID_AA64MMFR4_EL1_E2H0_WIDTH
> > cmp x1, #0
> > - b.ge .LnVHE_\@
> > +// b.ge .LnVHE_\@
> > 
> > orr x0, x0, #HCR_E2H
> > .LnVHE_\@:
> > 
> 
> With this change Guest OS starts the boot process.
> 
> [ 0.000000] CPU features: detected: Virtualization Host Extensions
> [ 6.998696] SMP: Total of 4 processors activated.
> [ 7.257045] CPU: All CPU(s) started at EL2
> [ 185.781062] SVE: maximum available vector length 16 bytes per vector
> [ 186.104159] SVE: default vector length 16 bytes per vector

Great. That confirms my suspicion that we cannot advertise VHE on CPUs
that do not have FEAT_FGT. Oh well. I'll post a patch to forbid VHE
guests on these machines.

> I also observed something interesting.
> When PSCI method is set to HVC, I get a kernel panic.
> This wasn’t a problem in all other test cases.

Well, that's completely expected. HVC is handled by EL2. You run at
EL2. You end-up calling yourself, which you don't really expect.

SMC is the way for EL2 guests, as if they were on bare-metal.

> On a side note, thank you for giving me some pointers.
> With HAS_EL2_E2H the original Guest OS boots fine in nVHE.
> 
> [    0.204744] CPU: All CPU(s) started at EL2
> [   18.483737] SVE: maximum available vector length 16 bytes per vector
> [   18.500959] SVE: default vector length 16 bytes per vector
> [   25.999475] kvm [1]: Hyp nVHE mode initialized successfully

Yup. I'll kill the ability to run VHE guests on crappy old HW then.

Thanks for giving it a go!

	M.

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