Re: [PATCH v2 2/8] arm: KVM: Add optimized PIPT icache flushing

[Marc Zyngier <marc.zyngier@arm.com>]

Hi Mark,

On 20/10/17 17:27, Mark Rutland wrote:
> Hi Marc,
> 
> On Fri, Oct 20, 2017 at 04:48:58PM +0100, Marc Zyngier wrote:
>> @@ -181,18 +185,40 @@ static inline void __invalidate_icache_guest_page(struct kvm_vcpu *vcpu,
>>  		return;
>>  	}
>>  
>> -	/* PIPT cache. As for the d-side, use a temporary kernel mapping. */
>> +	/*
>> +	 * CTR IminLine contains Log2 of the number of words in the
>> +	 * cache line, so we can get the number of words as
>> +	 * 2 << (IminLine - 1).  To get the number of bytes, we
>> +	 * multiply by 4 (the number of bytes in a 32-bit word), and
>> +	 * get 4 << (IminLine).
>> +	 */
>> +	iclsz = 4 << (read_cpuid(CPUID_CACHETYPE) & 0xf);
>> +
>>  	while (size) {
>>  		void *va = kmap_atomic_pfn(pfn);
>> +		void *end = va + PAGE_SIZE;
>> +		void *addr = va;
>>  
>> -		__cpuc_coherent_user_range((unsigned long)va,
>> -					   (unsigned long)va + PAGE_SIZE);
>> +		do {
>> +			write_sysreg(addr, ICIMVAU);
>> +			addr += iclsz;
>> +		} while (addr < end);
>> +
>> +		dsb(ishst);
> 
> I believe this needs to be ISH rather than ISHST.
> 
> Per, ARM DDI 0487B.b, page G3-4701, "G3.4 AArch32 cache and branch
> predictor support":
> 
>     A DSB or DMB instruction intended to ensure the completion of cache
>     maintenance instructions or branch predictor instructions must have
>     an access type of both loads and stores.

Right. This actually comes from 6abdd491698a ("ARM: mm: use
inner-shareable barriers for TLB and user cache operations"), and the
ARMv7 ARM doesn't mention any of this.

My take is that we want to be consistent. Given that KVM/ARM on 32bit is
basically ARMv7 only, I'd rather keep the ST version of the barrier
here, and change it everywhere if/when someone decides to support a
32bit kernel on ARMv8 (yes, we already do as a guest, but it doesn't
seem to really matter so far).

Thoughts?

	M.
-- 
Jazz is not dead. It just smells funny...