Re: [PATCH v5 07/10] KVM: arm64: Treat CTR_EL0 as a VM feature ID register

[Marc Zyngier <maz@kernel.org>]

Hi Shameer,

On Mon, 09 Sep 2024 16:19:54 +0100,
Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com> wrote:
> 
> Hi Oliver/Sebastian,
> 
> > -----Original Message-----
> > From: Oliver Upton <oliver.upton@linux.dev>
> > Sent: Wednesday, June 19, 2024 6:41 PM
> > To: kvmarm@lists.linux.dev
> > Cc: Marc Zyngier <maz@kernel.org>; James Morse
> > <james.morse@arm.com>; Suzuki K Poulose <suzuki.poulose@arm.com>;
> > yuzenghui <yuzenghui@huawei.com>; kvm@vger.kernel.org; Sebastian Ott
> > <sebott@redhat.com>; Shaoqin Huang <shahuang@redhat.com>; Eric Auger
> > <eric.auger@redhat.com>; Oliver Upton <oliver.upton@linux.dev>
> > Subject: [PATCH v5 07/10] KVM: arm64: Treat CTR_EL0 as a VM feature ID
> > register
> 
> [...] 
>  
> > @@ -2487,7 +2490,10 @@ static const struct sys_reg_desc sys_reg_descs[] =
> > {
> >  	{ SYS_DESC(SYS_CCSIDR2_EL1), undef_access },
> >  	{ SYS_DESC(SYS_SMIDR_EL1), undef_access },
> >  	{ SYS_DESC(SYS_CSSELR_EL1), access_csselr, reset_unknown,
> > CSSELR_EL1 },
> > -	{ SYS_DESC(SYS_CTR_EL0), access_ctr },
> > +	ID_WRITABLE(CTR_EL0, CTR_EL0_DIC_MASK |
> > +			     CTR_EL0_IDC_MASK |
> > +			     CTR_EL0_DminLine_MASK |
> > +			     CTR_EL0_IminLine_MASK),
> 
> (Sorry if this was discussed earlier, but I couldn't locate it anywhere.)
> 
> Is there a reason why we can't make the L1Ip writable as well here?
> We do have hardware that reports VIPT and PIPT for L11p.
> 
> The comment here states,
> https://elixir.bootlin.com/linux/v6.11-rc7/source/arch/arm64/kernel/cpufeature.c#L489
> 
> " If we have differing I-cache policies, report it as the weakest - VIPT."
> 
> Does this also mean it is safe to downgrade the PIPT to VIPT for Guest as well?

It should be safe, as a PIPT CMO always does at least the same as
VIPT, and potentially more if there is aliasing.

Thanks,

	M.

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