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

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Message-ID: <20180704155231.GP4828@arm.com>
Date:   Wed, 4 Jul 2018 16:52:31 +0100
From:   Will Deacon <will.deacon@....com>
To:     Julien Grall <julien.grall@....com>
Cc:     Suzuki K Poulose <suzuki.poulose@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        james.morse@....com, marc.zyngier@....com, cdall@...nel.org,
        eric.auger@...hat.com, catalin.marinas@....com,
        punit.agrawal@....com, qemu-devel@...gnu.org
Subject: Re: [kvmtool test PATCH 22/24] kvmtool: arm64: Add support for guest
 physical address size

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@....com>
> >>---
> >>  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.

Will