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

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Date:   Wed, 4 Jul 2018 16:00:11 +0100
From:   Julien Grall <julien.grall@....com>
To:     Will Deacon <will.deacon@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>
Cc:     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

Hi,

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.

Cheers,

[1] 
http://archive.armlinux.org.uk/lurker/message/20180510.140428.1c295b5b.en.html

> 
> Will
> 

-- 
Julien Grall