linux-kernel - Re: [PATCH v5 7/7] KVM: arm64: Normalize cache configuration

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Message-ID: <Y7c3AZJhlxKcc7y2@google.com>
Date:   Thu, 5 Jan 2023 20:45:53 +0000
From:   Oliver Upton <oliver.upton@...ux.dev>
To:     Akihiko Odaki <akihiko.odaki@...nix.com>
Cc:     Mark Brown <broonie@...nel.org>, Marc Zyngier <maz@...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 v5 7/7] KVM: arm64: Normalize cache configuration

Hi Akihiko,

On Fri, Dec 30, 2022 at 06:54:52PM +0900, Akihiko Odaki wrote:

[...]

> @@ -417,6 +418,9 @@ struct kvm_vcpu_arch {
>  		u64 last_steal;
>  		gpa_t base;
>  	} steal;
> +
> +	/* Per-vcpu CCSIDR override or NULL */
> +	u32 *ccsidr;

I don't believe we need to store this per-vCPU. Of course, it is
possible to userspace to observe heterogeneous cache topologies
depending on what core the KVM_GET_ONE_REG ioctl is handled on.

WDYT about keeping track of this per-VM? The value should be whatever
was last written by userspace. To avoid breaking userspace this would
also need to allow mismatched writes (i.e. vCPU0 has a different
configuration than vCPU1).

[...]

> +static u8 get_min_cache_line_size(u32 csselr)

It would be nice to have a comment indicating that it returns
Log2(line_size), not line size outright.

> +{
> +	u64 ctr_el0;
> +	int field;
> +
> +	ctr_el0 = read_sanitised_ftr_reg(SYS_CTR_EL0);
> +	field = csselr & CSSELR_EL1_InD ? CTR_EL0_IminLine_SHIFT : CTR_EL0_DminLine_SHIFT;
> +
> +	return cpuid_feature_extract_unsigned_field(ctr_el0, field) - 2;

This probably deserves a comment describing that CTR_EL0 represents line
size in units of words, not bytes. Furthermore, a subtle reminder on the
log transformation being done here would help also.

> +}
> +
>  /* Which cache CCSIDR represents depends on CSSELR value. */
> -static u32 get_ccsidr(u32 csselr)
> +static u32 get_ccsidr(struct kvm_vcpu *vcpu, u32 csselr)
>  {
> -	u32 ccsidr;
> +	if (vcpu->arch.ccsidr)
> +		return vcpu->arch.ccsidr[csselr];
>  
> -	/* Make sure noone else changes CSSELR during this! */
> -	local_irq_disable();
> -	write_sysreg(csselr, csselr_el1);
> -	isb();
> -	ccsidr = read_sysreg(ccsidr_el1);
> -	local_irq_enable();
> +	/*
> +	 * Fabricate a CCSIDR value as the overriding value does not exist.
> +	 * The real CCSIDR value will not be used as it can vary by the
> +	 * physical CPU which the vcpu currently resides in.
> +	 *
> +	 * The line size is determined with get_min_cache_line_size(), which
> +	 * should be valid for all CPUs even if they have different cache
> +	 * configuration.
> +	 *
> +	 * The associativity bits are cleared, meaning the geometry of all data
> +	 * and unified caches (which are guaranteed to be PIPT and thus
> +	 * non-aliasing) are 1 set and 1 way.
> +	 * Guests should not be doing cache operations by set/way at all, and
> +	 * for this reason, we trap them and attempt to infer the intent, so
> +	 * that we can flush the entire guest's address space at the appropriate
> +	 * time. The exposed geometry minimizes the number of the traps.
> +	 * [If guests should attempt to infer aliasing properties from the
> +	 * geometry (which is not permitted by the architecture), they would
> +	 * only do so for virtually indexed caches.]
> +	 *
> +	 * We don't check if the cache level exists as it is allowed to return
> +	 * an UNKNOWN value if not.
> +	 */
> +	return get_min_cache_line_size(csselr) << CCSIDR_EL1_LineSize_SHIFT;

I don't believe this is correct. The value of CCSIDR_EL1.LineSize is
actually:

   Log2(Number of bytes in cache line) - 4 (from DDI0487I.a D17.2.26)


With that, I'd expect the following instead:

	line_size = get_min_cache_line_size(csselr);
	return FIELD_PREP(CCSIDR_EL1_LineSize_MASK, line_size - 4);

--
Thanks,
Oliver