Re: [PATCH] ARM/arm64: KVM: test properly for a PTE's uncachedness

From: Christoffer Dall <christoffer.dall@linaro.org>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: marc.zyngier@arm.com, kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH] ARM/arm64: KVM: test properly for a PTE's uncachedness
Date: Mon, 9 Nov 2015 17:21:59 +0100	[thread overview]
Message-ID: <20151109162159.GA12968@cbox> (raw)
In-Reply-To: <1446810188-13727-1-git-send-email-ard.biesheuvel@linaro.org>

On Fri, Nov 06, 2015 at 12:43:08PM +0100, Ard Biesheuvel wrote:
> The open coded tests for checking whether a PTE maps a page as
> uncached use a flawed 'pte_val(xxx) & CONST != CONST' pattern,
> which is not guaranteed to work since the type of a mapping is an
> index into the MAIR table, not a set of mutually exclusive bits.
> 
> Considering that, on arm64, the S2 type definitions use the following
> MAIR indexes
> 
>     #define MT_S2_NORMAL            0xf
>     #define MT_S2_DEVICE_nGnRE      0x1
> 
> we have been getting lucky merely because the S2 device mappings also
> have the PTE_UXN bit set, which means that a device PTE still does not
> equal a normal PTE after masking with the former type.
> 
> Instead, implement proper checking against the MAIR indexes that are
> known to define uncached memory attributes.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  arch/arm/include/asm/kvm_mmu.h   | 11 +++++++++++
>  arch/arm/kvm/mmu.c               |  5 ++---
>  arch/arm64/include/asm/kvm_mmu.h | 12 ++++++++++++
>  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 405aa1883307..422973835d41 100644
> --- a/arch/arm/include/asm/kvm_mmu.h
> +++ b/arch/arm/include/asm/kvm_mmu.h
> @@ -279,6 +279,17 @@ static inline void __kvm_extend_hypmap(pgd_t *boot_hyp_pgd,
>  				       pgd_t *merged_hyp_pgd,
>  				       unsigned long hyp_idmap_start) { }
>  
> +static inline bool __kvm_pte_is_uncached(pte_t pte)
> +{
> +	switch (pte_val(pte) & L_PTE_MT_MASK) {
> +	case L_PTE_MT_UNCACHED:
> +	case L_PTE_MT_BUFFERABLE:
> +	case L_PTE_MT_DEV_SHARED:
> +		return true;
> +	}

so PTEs created by setting PAGE_S2_DEVICE will end up hitting in one of
these because L_PTE_S2_MT_DEV_SHARED is the same as L_PTE_MT_BUFFERABLE
for stage-2 mappings and PAGE_HYP_DEVICE end up using
L_PTE_MT_DEV_SHARED.

Totally obvious.

> +	return false;
> +}
> +
>  #endif	/* !__ASSEMBLY__ */
>  
>  #endif /* __ARM_KVM_MMU_H__ */
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 6984342da13d..eb9a06e3dbee 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -213,7 +213,7 @@ static void unmap_ptes(struct kvm *kvm, pmd_t *pmd,
>  			kvm_tlb_flush_vmid_ipa(kvm, addr);
>  
>  			/* No need to invalidate the cache for device mappings */
> -			if ((pte_val(old_pte) & PAGE_S2_DEVICE) != PAGE_S2_DEVICE)
> +			if (!__kvm_pte_is_uncached(old_pte))
>  				kvm_flush_dcache_pte(old_pte);
>  
>  			put_page(virt_to_page(pte));
> @@ -305,8 +305,7 @@ static void stage2_flush_ptes(struct kvm *kvm, pmd_t *pmd,
>  
>  	pte = pte_offset_kernel(pmd, addr);
>  	do {
> -		if (!pte_none(*pte) &&
> -		    (pte_val(*pte) & PAGE_S2_DEVICE) != PAGE_S2_DEVICE)
> +		if (!pte_none(*pte) && !__kvm_pte_is_uncached(*pte))
>  			kvm_flush_dcache_pte(*pte);
>  	} while (pte++, addr += PAGE_SIZE, addr != end);
>  }
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 61505676d085..5806f412a47a 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -302,5 +302,17 @@ static inline void __kvm_extend_hypmap(pgd_t *boot_hyp_pgd,
>  	merged_hyp_pgd[idmap_idx] = __pgd(__pa(boot_hyp_pgd) | PMD_TYPE_TABLE);
>  }
>  
> +static inline bool __kvm_pte_is_uncached(pte_t pte)
> +{
> +	switch (pte_val(pte) & PTE_ATTRINDX_MASK) {
> +	case PTE_ATTRINDX(MT_DEVICE_nGnRnE):
> +	case PTE_ATTRINDX(MT_DEVICE_nGnRE):
> +	case PTE_ATTRINDX(MT_DEVICE_GRE):
> +	case PTE_ATTRINDX(MT_NORMAL_NC):
> +		return true;
> +	}
> +	return false;
> +}
> +
>  #endif /* __ASSEMBLY__ */
>  #endif /* __ARM64_KVM_MMU_H__ */
> -- 
> 1.9.1
> 

Thanks for this patch, I'll queue it.

-Christoffer