Re: [PATCH v7 00/14] arm/mem_access: Walk guest page tables in SW if mem_access is active

[Sergej Proskurin <proskurin@sec.in.tum.de>]

Hi Julien,

> The patch belows solve my problem:
>
> diff --git a/xen/arch/arm/guest_walk.c b/xen/arch/arm/guest_walk.c
> index b258248322..6ca994e438 100644
> --- a/xen/arch/arm/guest_walk.c
> +++ b/xen/arch/arm/guest_walk.c
> @@ -112,7 +112,7 @@ static int guest_walk_sd(const struct vcpu *v,
>           * level translation table does not need to be page aligned.
>           */
>          mask = GENMASK(19, 12);
> -        paddr = (pte.walk.base << 10) | ((gva & mask) >> 10);
> +        paddr = ((paddr_t)pte.walk.base << 10) | ((gva & mask) >> 10);
>  
>          /* Access the guest's memory to read only one PTE. */
>          ret = access_guest_memory_by_ipa(d, paddr, &pte, sizeof(short_desc_t), false);
>
> This is because pte.walk.base is encoded on unsigned int:22 bits. A shift by 10 will not
> fit an integer, and my compiler seems to promote it to "signed long long". Hence the bogus
> address.
>


Thats quite an interesting phenomenon :) I have just played around with
this and it does indeed appear that the value is casted to a signed
result! What I don't yet understand is the following: An unsigned int
with the length of 22 bit should actually exactly fit an integer after a
left shift of 10 (or do I miss s.th.?).

Anyway, thanks for the patch! V8 containing this change will follow soon.

Thanks,
~Sergej



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