Re: [PATCH v2 21/22] KVM: MMU: mmio page fault support

Avi Kivity
2011-06-29T09:22:59Z

On 06/22/2011 05:36 PM, Xiao Guangrong wrote:
> The idea is from Avi:
>
> | We could cache the result of a miss in an spte by using a reserved bit, and
> | checking the page fault error code (or seeing if we get an ept violation or
> | ept misconfiguration), so if we get repeated mmio on a page, we don't need to
> | search the slot list/tree.
> | (https://lkml.org/lkml/2011/2/22/221)
>
> When the page fault is caused by mmio, we cache the info in the shadow page
> table, and also set the reserved bits in the shadow page table, so if the mmio
> is caused again, we can quickly identify it and emulate it directly
>
> Searching mmio gfn in memslots is heavy since we need to walk all memeslots, it
> can be reduced by this feature, and also avoid walking guest page table for
> soft mmu.
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 1319050..e69a47a 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -197,6 +197,41 @@ static u64 __read_mostly shadow_x_mask; /* mutual exclusive with nx_mask */
> static u64 __read_mostly shadow_user_mask;
> static u64 __read_mostly shadow_accessed_mask;
> static u64 __read_mostly shadow_dirty_mask;
> +static u64 __read_mostly shadow_mmio_mask = (0xffull<< 49 | 1ULL);

One bit is shifted out. And it will fail with 52-bit MAXPHYADDR.

Please in addition, set the xwr bits to an invalid pattern on EPT (there
is an MSR which specifies which patterns are valid; for example
execute-only or write-only are invalid). If all patterns are valid AND
MAXPHYADDR == 52, then just set the mask to 0 and it the optimization
will be disabled.

--
error compiling committee.c: too many arguments to function