Re: [Xen-devel] [PATCH V6 4/5] xen/mm: Clean up pfec handling in gva_to_gfn

["Han, Huaitong" <huaitong.han@xxxxxxxxx>]

On Tue, 2016-01-26 at 14:30 +0000, Tim Deegan wrote:
> Hi,
> 
> At 15:30 +0800 on 19 Jan (1453217458), Huaitong Han wrote:
> > At the moment, the pfec argument to gva_to_gfn has two functions:
> > 
> > * To inform guest_walk what kind of access is happenind
> > 
> > * As a value to pass back into the guest in the event of a fault.
> > 
> > Unfortunately this is not quite treated consistently: the
> > hvm_fetch_*
> > function will "pre-clear" the PFEC_insn_fetch flag before calling
> > gva_to_gfn; meaning guest_walk doesn't actually know whether a
> > given
> > access is an instruction fetch or not.  This works now, but will
> > cause
> > issues when pkeys are introduced, since guest_walk will need to
> > know
> > whether an access is an instruction fetch even if it doesn't return
> > PFEC_insn_fetch.
> > 
> > Fix this by making a clean separation for in and out
> > functionalities
> > of the pfec argument:
> > 
> > 1. Always pass in the access type to gva_to_gfn
> > 
> > 2. Filter out inappropriate access flags before returning from
> > gva_to_gfn.
> 
> This seems OK.  But can you please:
>  - Add this new adjustment once, in paging_gva_to_gfn(), instead of
>    adding it to each implementation; and
>  - Adjust the comment above the declaration of paging_gva_to_gfn() in
>    paging.h to describe this new behaviour.
Although adding adjustment in paging_gva_to_gfn can reduce code
duplication, adding it to each implementation is more readable, becasue
other sections of pfec are handled in each implementation.

> 
> Cheers,
> 
> Tim.
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