Re: [PATCH 04/15] KVM: MMU: cache mmio info on page fault path

[Marcelo Tosatti <mtosatti@redhat.com>]

On Tue, Jun 07, 2011 at 09:00:30PM +0800, Xiao Guangrong wrote:
> If the page fault is caused by mmio, we can cache the mmio info, later, we do
> not need to walk guest page table and quickly know it is a mmio fault while we
> emulate the mmio instruction
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
> ---
>  arch/x86/include/asm/kvm_host.h |    5 +++
>  arch/x86/kvm/mmu.c              |   21 +++++----------
>  arch/x86/kvm/mmu.h              |   23 +++++++++++++++++
>  arch/x86/kvm/paging_tmpl.h      |   21 ++++++++++-----
>  arch/x86/kvm/x86.c              |   52 ++++++++++++++++++++++++++++++--------
>  arch/x86/kvm/x86.h              |   36 +++++++++++++++++++++++++++
>  6 files changed, 126 insertions(+), 32 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index d167039..326af42 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -414,6 +414,11 @@ struct kvm_vcpu_arch {
>  	u64 mcg_ctl;
>  	u64 *mce_banks;
>  
> +	/* Cache MMIO info */
> +	u64 mmio_gva;
> +	unsigned access;
> +	gfn_t mmio_gfn;
> +
>  	/* used for guest single stepping over the given code position */
>  	unsigned long singlestep_rip;
>  

Why you're not implementing the original idea to cache the MMIO
attribute of an address into the spte?

That solution is wider reaching than a one-entry cache, and was proposed
to overcome large number of memslots.

If the access pattern switches between different addresses this one
solution is doomed.