linux-kernel - Re: [PATCH v5 10/10] KVM: arm64: np-guest CMOs with PMD

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Message-ID: <aC28TB_2Aul4s1T6@google.com>
Date: Wed, 21 May 2025 12:43:08 +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 v5 10/10] KVM: arm64: np-guest CMOs with PMD_SIZE fixmap

On Wed, May 21, 2025 at 12:01:26PM +0100, Marc Zyngier wrote:
> On Tue, 20 May 2025 09:52:01 +0100,
> Vincent Donnefort <vdonnefort@...gle.com> wrote:
> > 
> > With the introduction of stage-2 huge mappings in the pKVM hypervisor,
> > guest pages CMO is needed for PMD_SIZE size. Fixmap only supports
> > PAGE_SIZE and iterating over the huge-page is time consuming (mostly due
> > to TLBI on hyp_fixmap_unmap) which is a problem for EL2 latency.
> > 
> > Introduce a shared PMD_SIZE fixmap (hyp_fixblock_map/hyp_fixblock_unmap)
> > to improve guest page CMOs when stage-2 huge mappings are installed.
> > 
> > On a Pixel6, the iterative solution resulted in a latency of ~700us,
> > while the PMD_SIZE fixmap reduces it to ~100us.
> > 
> > Because of the horrendous private range allocation that would be
> > necessary, this is disabled for 64KiB pages systems.
> > 
> > Suggested-by: Quentin Perret <qperret@...gle.com>
> > Signed-off-by: Vincent Donnefort <vdonnefort@...gle.com>
> > Signed-off-by: Quentin Perret <qperret@...gle.com>
> > 
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index 1b43bcd2a679..2888b5d03757 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -59,6 +59,11 @@ typedef u64 kvm_pte_t;
> >  
> >  #define KVM_PHYS_INVALID		(-1ULL)
> >  
> > +#define KVM_PTE_TYPE			BIT(1)
> > +#define KVM_PTE_TYPE_BLOCK		0
> > +#define KVM_PTE_TYPE_PAGE		1
> > +#define KVM_PTE_TYPE_TABLE		1
> > +
> >  #define KVM_PTE_LEAF_ATTR_LO		GENMASK(11, 2)
> >  
> >  #define KVM_PTE_LEAF_ATTR_LO_S1_ATTRIDX	GENMASK(4, 2)
> > diff --git a/arch/arm64/kvm/hyp/include/nvhe/mm.h b/arch/arm64/kvm/hyp/include/nvhe/mm.h
> > index 230e4f2527de..6e83ce35c2f2 100644
> > --- a/arch/arm64/kvm/hyp/include/nvhe/mm.h
> > +++ b/arch/arm64/kvm/hyp/include/nvhe/mm.h
> > @@ -13,9 +13,11 @@
> >  extern struct kvm_pgtable pkvm_pgtable;
> >  extern hyp_spinlock_t pkvm_pgd_lock;
> >  
> > -int hyp_create_pcpu_fixmap(void);
> > +int hyp_create_fixmap(void);
> >  void *hyp_fixmap_map(phys_addr_t phys);
> >  void hyp_fixmap_unmap(void);
> > +void *hyp_fixblock_map(phys_addr_t phys, size_t *size);
> > +void hyp_fixblock_unmap(void);
> >  
> >  int hyp_create_idmap(u32 hyp_va_bits);
> >  int hyp_map_vectors(void);
> > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > index 1490820b9ebe..962948534179 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > @@ -216,34 +216,42 @@ static void guest_s2_put_page(void *addr)
> >  	hyp_put_page(&current_vm->pool, addr);
> >  }
> >  
> > -static void clean_dcache_guest_page(void *va, size_t size)
> > +static void __apply_guest_page(void *va, size_t size,
> > +			       void (*func)(void *addr, size_t size))
> >  {
> >  	size += va - PTR_ALIGN_DOWN(va, PAGE_SIZE);
> >  	va = PTR_ALIGN_DOWN(va, PAGE_SIZE);
> >  	size = PAGE_ALIGN(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 map_size = PAGE_SIZE;
> > +		void *map;
> > +
> > +		if (size >= PMD_SIZE)
> > +			map = hyp_fixblock_map(__hyp_pa(va), &map_size);
> 
> You seem to consider that if size if PMD_SIZE (or more), then va must
> be PMD aligned. I don't think this is correct.
> 
> Such an iterator should start by doing PAGE_SIZEd operations until va
> is PMD-aligned. Only at this point can it perform PMD_SIZEd
> operations, until the remaining size is less than PMD_SIZE. And at
> that point, it's PAGE_SIZE all over again until the end.

Arg yes you're right :-\ 

Shall I respin a v6 with that fix or shall I wait a bit more?

> 
> Does that make sense to you?
> 
> 	M.
> 
> -- 
> Without deviation from the norm, progress is not possible.