Re: [kvmtool test PATCH 22/24] kvmtool: arm64: Add support for guest physical address size

[Julien Grall]

Hi Will,

On 04/07/18 16:52, Will Deacon wrote:
> On Wed, Jul 04, 2018 at 04:00:11PM +0100, Julien Grall wrote:
> > On 04/07/18 15:09, Will Deacon wrote:
> > > On Fri, Jun 29, 2018 at 12:15:42PM +0100, Suzuki K Poulose wrote:
> > > > Add an option to specify the physical address size used by this
> > > > VM.
> > > >
> > > > Signed-off-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
> > > > ---
> > > >   arm/aarch64/include/kvm/kvm-config-arch.h | 5 ++++-
> > > >   arm/include/arm-common/kvm-config-arch.h  | 1 +
> > > >   2 files changed, 5 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arm/aarch64/include/kvm/kvm-config-arch.h b/arm/aarch64/include/kvm/kvm-config-arch.h
> > > > index 04be43d..dabd22c 100644
> > > > --- a/arm/aarch64/include/kvm/kvm-config-arch.h
> > > > +++ b/arm/aarch64/include/kvm/kvm-config-arch.h
> > > > @@ -8,7 +8,10 @@
> > > >   			"Create PMUv3 device"),				\
> > > >   	OPT_U64('\0', "kaslr-seed", &(cfg)->kaslr_seed,			\
> > > >   			"Specify random seed for Kernel Address Space "	\
> > > > -			"Layout Randomization (KASLR)"),
> > > > +			"Layout Randomization (KASLR)"),		\
> > > > +	OPT_INTEGER('\0', "phys-shift", &(cfg)->phys_shift,		\
> > > > +			"Specify maximum physical address size (not "	\
> > > > +			"the amount of memory)"),
> > >
> > > Given that this is a shift value, I think the help message could be more
> > > informative. Something like:
> > >
> > > 	"Specify maximum number of bits in a guest physical address"
> > >
> > > I think I'd actually leave out any mention of memory, because this does
> > > actually have an effect on the amount of addressable memory in a way that I
> > > don't think we want to describe in half of a usage message line :)
> > Is there any particular reasons to expose this option to the user?
> >
> > I have recently sent a series to allow the user to specify the position
> > of the RAM [1]. With that series in mind, I think the user would not really
> > need to specify the maximum physical shift. Instead we could automatically
> > find it.
>
> Marc makes a good point that it doesn't help for MMIO regions, so I'm trying
> to understand whether we can do something differently there and avoid
> sacrificing the type parameter.

I am not sure to understand this. kvmtools knows the memory layout 
(including MMIOs) of the guest, so couldn't it guess the maximum 
physical shift for that?

Cheers,

--
Julien Grall