linux-kernel - Re: [PATCH v7 7/7] KVM: arm64: Normalize cache configuration

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Date:   Sat, 21 Jan 2023 18:15:23 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Akihiko Odaki <akihiko.odaki@...nix.com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org,
        kvmarm@...ts.linux.dev, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        James Morse <james.morse@....com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        asahi@...ts.linux.dev, Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Sven Peter <sven@...npeter.dev>,
        Hector Martin <marcan@...can.st>
Subject: Re: [PATCH v7 7/7] KVM: arm64: Normalize cache configuration

Hey Marc,

On Sat, Jan 21, 2023 at 12:02:03PM +0000, Marc Zyngier wrote:
> On Thu, 19 Jan 2023 19:46:16 +0000, Oliver Upton <oliver.upton@...ux.dev> wrote:
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index 459e6d358dab..b6228f7d1d8d 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -148,17 +148,19 @@ static u32 get_ccsidr(struct kvm_vcpu *vcpu, u32 csselr)
> >  
> >  static int set_ccsidr(struct kvm_vcpu *vcpu, u32 csselr, u32 val)
> >  {
> > -	u8 line_size = FIELD_GET(CCSIDR_EL1_LineSize, val);
> > +	u8 line_size = SYS_FIELD_GET(CCSIDR_EL1, LineSize, val);
> > +	u32 cur = get_ccsidr(vcpu, csselr);
> > +	u8 min_line_size = SYS_FIELD_GET(CCSIDR_EL1, LineSize, cur);
> >  	u32 *ccsidr = vcpu->arch.ccsidr;
> >  	u32 i;
> >  
> > -	if ((val & CCSIDR_EL1_RES0) || line_size < get_min_cache_line_size(csselr))
> > +	if (cur == val)
> > +		return 0;
> > +
> > +	if ((val & CCSIDR_EL1_RES0) || line_size < min_line_size)
> >  		return -EINVAL;
> 
> This doesn't look right. You're comparing the value userspace is
> trying to set for a given level with the value that is already set for
> that level, and forbid the cache line size to be smaller. It works if
> no value has been set yet (you fallback to something derived from
> CTR_EL0), but this fails if userspace does multiple writes.

Good catch, I tried to skip over the unit/field conversions by doing this
but it has the consequence of not working as expected for multiple writes.

> The original check is against CTR_EL0, which makes absolute sense
> because we want to check across the whole hierarchy. It is just that
> the original code has two bugs:
> 
> - It fails to convert the CCSIDR_EL1.LineSize value to a number of
>   words (the missing +4). Admire how the architecture is actively
>   designed to be hostile to SW by providing two different formats for
>   the cache line size, none of which is in... bytes.
> 
> - It passes the full CSSELR value to get_min_cache_line_size(), while
>   this function wants a bool... Yes, there are times where you'd want
>   a stronger type system (did anyone say Rust? ;-)

Hey now, if you say it enough times people are going to start getting
ideas ;-P

> I propose that we fold something like the patch below in instead
> (tested with get-reg-list).

Agreed, I've backed out my diff and applied yours. Pushed (with force!)
to my repo now, PTAL.

--
Thanks,
Oliver