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

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Message-ID: <df1f5846-dcfe-5b60-bfe3-54d93698831d@daynix.com>
Date:   Sat, 7 Jan 2023 19:04:03 +0900
From:   Akihiko Odaki <akihiko.odaki@...nix.com>
To:     Oliver Upton <oliver.upton@...ux.dev>
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

On 2023/01/06 5:45, Oliver Upton wrote:
> 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).
A user would expect KVM_SET_ONE_REG will be per-VM so semantically it 
should be per-VM. Also, it should be noted that this change is to allow 
migration from older kernels; an older kernel can set different values 
for CCSIDR if the host has heterogeneous cache though I think nobody 
with big.LITTLE system wants to migrate VMs.

If you care only practicality but semantics, probably you can just 
ignore writes to CCSIDR from the userspace. I don't think any guest will 
care even if CCSIDR values change after migration. Either way is fine 
for me.

> 
> [...]
> 
>> +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

I added a comment to get_min_cache_line_size() with v6. Hopefully it 
will clarify the intention.

Regards,
Akihiko Odaki