Re: [PATCH] KVM: arm/arm64: Revert to old way of checking for device mapping in stage2_flush_ptes().

[Christoffer Dall <christoffer.dall@linaro.org>]

On Tue, Dec 01, 2015 at 04:03:52PM +0300, Pavel Fedin wrote:
> This function takes stage-II physical addresses (A.K.A. IPA), on input, not
> real physical addresses. This causes kvm_is_device_pfn() to return wrong
> values, depending on how much guest and host memory maps match. This
> results in completely broken KVM on some boards. The problem has been
> caught on Samsung proprietary hardware.
> 
> Cc: stable@vger.kernel.org

cc'ing stable doesn't make sense here as the bug was introduced in
v4.4-rc3 and we didn't release v4.4 yet...

> Fixes: e6fab5442345 ("ARM/arm64: KVM: test properly for a PTE's uncachedness")
> 
> Signed-off-by: Pavel Fedin <p.fedin@samsung.com>
> ---
>  arch/arm/kvm/mmu.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 7dace90..51ad98f 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -310,7 +310,8 @@ static void stage2_flush_ptes(struct kvm *kvm, pmd_t *pmd,
>  
>  	pte = pte_offset_kernel(pmd, addr);
>  	do {
> -		if (!pte_none(*pte) && !kvm_is_device_pfn(__phys_to_pfn(addr)))
> +		if (!pte_none(*pte) &&
> +		    (pte_val(*pte) & PAGE_S2_DEVICE) != PAGE_S2_DEVICE)
>  			kvm_flush_dcache_pte(*pte);
>  	} while (pte++, addr += PAGE_SIZE, addr != end);
>  }

You are right that there was a bug in the fix, but your fix is not the
right one.

Either we have to apply an actual mask and the compare against the value
(yes, I know, because of the UXN bit we get lucky so far, but that's too
brittle), or we should do a translation fo the gfn to a pfn.  Is there
anything preventing us to do the following?

if (!pte_none(*pte) && !kvm_is_device_pfn(pte_pfn(*pte)))

-Christoffer