Re: [Xen-devel] [PATCH v5 3/4] x86/mm: use cache in guest_walk_tables()

["Durrant, Paul" <pdurrant@xxxxxxxxxxxx>]

> -----Original Message-----
> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of Jan 
> Beulich
> Sent: 03 March 2020 10:19
> To: xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Roger Pau Monné 
> <roger.pau@xxxxxxxxxx>; Tim Deegan
> <tim@xxxxxxx>; Wei Liu <wl@xxxxxxx>; Paul Durrant <paul@xxxxxxx>
> Subject: [EXTERNAL][Xen-devel] [PATCH v5 3/4] x86/mm: use cache in 
> guest_walk_tables()
> 
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> 
> Emulation requiring device model assistance uses a form of instruction
> re-execution, assuming that the second (and any further) pass takes
> exactly the same path. This is a valid assumption as far as use of CPU
> registers goes (as those can't change without any other instruction
> executing in between [1]), but is wrong for memory accesses. In
> particular it has been observed that Windows might page out buffers
> underneath an instruction currently under emulation (hitting between two
> passes). If the first pass translated a linear address successfully, any
> subsequent pass needs to do so too, yielding the exact same translation.
> To guarantee this, leverage the caching that now backs HVM insn
> emulation.
> 
> [1] Other than on actual hardware, actions like
>     XEN_DOMCTL_sethvmcontext, XEN_DOMCTL_setvcpucontext,
>     VCPUOP_initialise, INIT, or SIPI issued against the vCPU can occur
>     while the vCPU is blocked waiting for a device model to return data.
>     In such cases emulation now gets canceled, though, and hence re-
>     execution correctness is unaffected.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Paul Durrant <pdurrant@xxxxxxxx>
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