linux-kernel - Re: [PATCH v4 10/10] KVM: arm64: np-guest CMOs with PMD

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Message-ID: <aCd94sBdThAj6jT9@google.com>
Date: Fri, 16 May 2025 19:03:14 +0100
From: Vincent Donnefort <vdonnefort@...gle.com>
To: Marc Zyngier <maz@...nel.org>
Cc: oliver.upton@...ux.dev, joey.gouly@....com, suzuki.poulose@....com,
	yuzenghui@...wei.com, catalin.marinas@....com, will@...nel.org,
	qperret@...gle.com, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org,
	kernel-team@...roid.com
Subject: Re: [PATCH v4 10/10] KVM: arm64: np-guest CMOs with PMD_SIZE fixmap

> > +}
> > +
> >  static void clean_dcache_guest_page(void *va, size_t size)
> >  {
> >  	WARN_ON(!PAGE_ALIGNED(size));
> >  
> >  	while (size) {
> > -		__clean_dcache_guest_page(hyp_fixmap_map(__hyp_pa(va)),
> > -					  PAGE_SIZE);
> > -		hyp_fixmap_unmap();
> > -		va += PAGE_SIZE;
> > -		size -= PAGE_SIZE;
> > +		size_t fixmap_size = size == PMD_SIZE ? size : PAGE_SIZE;
> > +		void *addr = __fixmap_guest_page(va, &fixmap_size);
> > +
> > +		__clean_dcache_guest_page(addr, fixmap_size);
> > +		__fixunmap_guest_page(fixmap_size);
> > +
> > +		size -= fixmap_size;
> > +		va += fixmap_size;
> 
> Can this ever be called with a *multiple* of PMD_SIZE? In this case
> you'd still end-up doing PAGE_SIZEd-bite CMOs until there is only
> PMD_SIZE left, ruining the optimisation.
> 
> I think this needs fixing.

So this can be only called with size either equal to PAGE_SIZE or PMD_SIZE. I
wasn't sure if it was worth to make it more generic than it needs.

But like for the first patch, I can make it more future-proof by handling size >
PMD_SIZE.

> 
> >  	}
> >  }
> >

[...]