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Re: [PATCH v3 04/12] target/ppc: Introduce ppc_hash64_use_vrma() helper

From: Paul Mackerras
Subject: Re: [PATCH v3 04/12] target/ppc: Introduce ppc_hash64_use_vrma() helper
Date: Thu, 20 Feb 2020 13:41:06 +1100
User-agent: Mutt/1.10.1 (2018-07-13)

On Wed, Feb 19, 2020 at 11:06:20AM -0300, Fabiano Rosas wrote:
> David Gibson <address@hidden> writes:
> > When running guests under a hypervisor, the hypervisor obviously needs to
> > be protected from guest accesses even if those are in what the guest
> > considers real mode (translation off).  The POWER hardware provides two
> > ways of doing that: The old way has guest real mode accesses simply offset
> > and bounds checked into host addresses.  It works, but requires that a
> > significant chunk of the guest's memory - the RMA - be physically
> > contiguous in the host, which is pretty inconvenient.  The new way, known
> > as VRMA, has guest real mode accesses translated in roughly the normal way
> > but with some special parameters.
> >
> > In POWER7 and POWER8 the LPCR[VPM0] bit selected between the two modes, but
> > in POWER9 only VRMA mode is supported
> ... when translation is off, right? Because I see in the 3.0 ISA that
> LPCR[VPM1] is still there.

VRMA stands for virtual real mode area, and the "real mode" part
implies that translation is off.  VRMA is not used when translation is
on because then the CPU is not in real mode.

LPCR[VPM1] is indeed still there, but it is a bit different to VPM0
(or what VPM0 used to do); VPM1 doesn't change how translation is
done, just what happens on a fault.


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