linux-kernel - Re: [PATCH 4/7] KVM: MMU: document mmio page fault

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Message-ID: <51C19FC6.9020001@redhat.com>
Date:	Wed, 19 Jun 2013 14:10:46 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	gleb@...hat.com, avi.kivity@...il.com, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 4/7] KVM: MMU: document mmio page fault

Il 19/06/2013 11:09, Xiao Guangrong ha scritto:
> Document it to Documentation/virtual/kvm/mmu.txt
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> ---
>  Documentation/virtual/kvm/mmu.txt | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
> index 5a6b2e2..4fb442b 100644
> --- a/Documentation/virtual/kvm/mmu.txt
> +++ b/Documentation/virtual/kvm/mmu.txt
> @@ -270,14 +270,21 @@ This is the most complicated event.  The cause of a page fault can be:
>  
>  Handling a page fault is performed as follows:
>  
> + - if the RSV bit of the error code is set, the page fault is caused by guest
> +   accessing MMIO, walk shadow page table to get the last spte where the mmio
> +   information is stored and cache the information to vcpu->arch.mmio_gva,
> +   vcpu->arch.access and vcpu->arch.mmio_gfn then call the emulator to emulate
> +   the instruction who will get the benefit from the cached mmio info

+ - if the RSV bit of the error code is set, the page fault is caused by guest
+   accessing MMIO and cached MMIO information is available.
+   - walk shadow page table
+   - cache the information to vcpu->arch.mmio_gva, vcpu->arch.access and
+     vcpu->arch.mmio_gfn, and call the emulator

>   - if needed, walk the guest page tables to determine the guest translation
>     (gva->gpa or ngpa->gpa)
>     - if permissions are insufficient, reflect the fault back to the guest
>   - determine the host page
> -   - if this is an mmio request, there is no host page; call the emulator
> -     to emulate the instruction instead
> +   - if this is an mmio request, there is no host page; cache the info to
> +     vcpu->arch.mmio_gva, vcpu->arch.access and vcpu->arch.mmio_gfn
>   - walk the shadow page table to find the spte for the translation,
>     instantiating missing intermediate page tables as necessary
> +   - If this is an mmio request, cache the mmio info to the spte and set some
> +     reserved bits on the spte

Added "(see callers of kvm_mmu_set_mmio_spte_mask)".  Not really related, but
just came to my mind: perhaps we can have a section on A/D bits too.

Paolo

>   - try to unsynchronize the page
>     - if successful, we can let the guest continue and modify the gpte
>   - emulate the instruction
> 

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