linux-kernel - Re: [RFC 2/4] KVM: arm64: Support dirty page tracking for PUD hugepages

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Message-ID: <20180206145508.GC23160@cbox>
Date:   Tue, 6 Feb 2018 15:55:08 +0100
From:   Christoffer Dall <christoffer.dall@...aro.org>
To:     Punit Agrawal <punit.agrawal@....com>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, suzuki.poulose@....com,
        Marc Zyngier <marc.zyngier@....com>
Subject: Re: [RFC 2/4] KVM: arm64: Support dirty page tracking for PUD
 hugepages

On Wed, Jan 10, 2018 at 07:07:27PM +0000, Punit Agrawal wrote:
> In preparation for creating PUD hugepages at stage 2, add support for
> write protecting PUD hugepages when they are encountered. Write
> protecting guest tables is used to track dirty pages when migrating VMs.
> 
> Also, provide trivial implementations of required kvm_s2pud_* helpers to
> allow code to compile on arm32.
> 
> Signed-off-by: Punit Agrawal <punit.agrawal@....com>
> Cc: Christoffer Dall <christoffer.dall@...aro.org>
> Cc: Marc Zyngier <marc.zyngier@....com>
> ---
>  arch/arm/include/asm/kvm_mmu.h   |  9 +++++++++
>  arch/arm64/include/asm/kvm_mmu.h | 10 ++++++++++
>  virt/kvm/arm/mmu.c               |  9 ++++++---
>  3 files changed, 25 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
> index fa6f2174276b..3fbe919b9181 100644
> --- a/arch/arm/include/asm/kvm_mmu.h
> +++ b/arch/arm/include/asm/kvm_mmu.h
> @@ -103,6 +103,15 @@ static inline bool kvm_s2pmd_readonly(pmd_t *pmd)
>  	return (pmd_val(*pmd) & L_PMD_S2_RDWR) == L_PMD_S2_RDONLY;
>  }
>  
> +static inline void kvm_set_s2pud_readonly(pud_t *pud)
> +{
> +}
> +
> +static inline bool kvm_s2pud_readonly(pud_t *pud)
> +{
> +	return true;

why true?  Shouldn't this return the pgd's readonly value, strictly
speaking, or if we rely on this never being called, have  VM_BUG_ON() ?

In any case, a comment explaining why we unconditionally return true
would be nice.

> +}
> +
>  static inline bool kvm_page_empty(void *ptr)
>  {
>  	struct page *ptr_page = virt_to_page(ptr);
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 672c8684d5c2..dbfd18e08cfb 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -201,6 +201,16 @@ static inline bool kvm_s2pmd_readonly(pmd_t *pmd)
>  	return kvm_s2pte_readonly((pte_t *)pmd);
>  }
>  
> +static inline void kvm_set_s2pud_readonly(pud_t *pud)
> +{
> +	kvm_set_s2pte_readonly((pte_t *)pud);
> +}
> +
> +static inline bool kvm_s2pud_readonly(pud_t *pud)
> +{
> +	return kvm_s2pte_readonly((pte_t *)pud);
> +}
> +
>  static inline bool kvm_page_empty(void *ptr)
>  {
>  	struct page *ptr_page = virt_to_page(ptr);
> diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> index 9dea96380339..02eefda5d71e 100644
> --- a/virt/kvm/arm/mmu.c
> +++ b/virt/kvm/arm/mmu.c
> @@ -1155,9 +1155,12 @@ static void  stage2_wp_puds(pgd_t *pgd, phys_addr_t addr, phys_addr_t end)
>  	do {
>  		next = stage2_pud_addr_end(addr, end);
>  		if (!stage2_pud_none(*pud)) {
> -			/* TODO:PUD not supported, revisit later if supported */
> -			BUG_ON(stage2_pud_huge(*pud));
> -			stage2_wp_pmds(pud, addr, next);
> +			if (stage2_pud_huge(*pud)) {
> +				if (!kvm_s2pud_readonly(pud))
> +					kvm_set_s2pud_readonly(pud);
> +			} else {
> +				stage2_wp_pmds(pud, addr, next);
> +			}
>  		}
>  	} while (pud++, addr = next, addr != end);
>  }
> -- 
> 2.15.1
> 

Otherwise:

Reviewed-by: Christoffer Dall <christoffer.dall@...aro.org>