linux-kernel - Re: [PATCH v2 1/2] KVM: arm64: Allow userspace to change ID_AA64PFR1

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Message-ID: <f73cabc6-8c48-4c12-9d60-269e50df41f3@redhat.com>
Date: Fri, 21 Jun 2024 14:17:57 +0800
From: Shaoqin Huang <shahuang@...hat.com>
To: Marc Zyngier <maz@...nel.org>
Cc: Oliver Upton <oliver.upton@...ux.dev>, kvmarm@...ts.linux.dev,
 James Morse <james.morse@....com>, Suzuki K Poulose
 <suzuki.poulose@....com>, Zenghui Yu <yuzenghui@...wei.com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] KVM: arm64: Allow userspace to change
 ID_AA64PFR1_EL1

Hi Marc,

On 6/18/24 15:39, Marc Zyngier wrote:
> On Tue, 18 Jun 2024 07:38:06 +0100,
> Shaoqin Huang <shahuang@...hat.com> wrote:
>>
>> Allow userspace to change the guest-visible value of the register with
>> some severe limitation:
>>
>>    - No changes to features not virtualized by KVM (MPAM_frac, RAS_frac)
>> ---
>>   arch/arm64/kvm/sys_regs.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>> index 22b45a15d068..bead81867bce 100644
>> --- a/arch/arm64/kvm/sys_regs.c
>> +++ b/arch/arm64/kvm/sys_regs.c
>> @@ -2306,7 +2306,8 @@ static const struct sys_reg_desc sys_reg_descs[] = {
>>   		   ID_AA64PFR0_EL1_GIC |
>>   		   ID_AA64PFR0_EL1_AdvSIMD |
>>   		   ID_AA64PFR0_EL1_FP), },
>> -	ID_SANITISED(ID_AA64PFR1_EL1),
>> +	ID_WRITABLE(ID_AA64PFR1_EL1, ~(ID_AA64PFR1_EL1_RAS_frac |
>> +				       ID_AA64PFR1_EL1_MPAM_frac)),
>>   	ID_UNALLOCATED(4,2),
>>   	ID_UNALLOCATED(4,3),
>>   	ID_WRITABLE(ID_AA64ZFR0_EL1, ~ID_AA64ZFR0_EL1_RES0),
> 
> This isn't a valid patch.
> 
> Furthermore, how about all the other features that may or may not be
> currently handled by KVM? Please see [1] and make sure that all
> existing fields have a known behaviour (a combination of masked,
> preserved, capped, writable or read-only).
> 
> I can at least see problems with MTE_frac and MTEX, plus all the other
> things that KVM doesn't know how to save/restore (THE, GCS, NMI...).
> 
> What I asked you to handle the whole register, I really meant it.

I currently only found the BT and SSBS fields can be written without any 
unknown behavior.

All other fields in the ID_AA64PFR1_EL1 are either not supported by KVM 
or the field involved with other register and KVM don't know how to 
handle them.

I'm not sure if this is right. So I want to ask about your opinion about 
how to allow each of the field to be writable in the ID_AA64PFR1_EL1 
register?

Thanks,
Shaoqin

> 
> 	M.
> 
> [1] https://developer.arm.com/documentation/ddi0601/2024-03/AArch64-Registers/ID-AA64PFR1-EL1--AArch64-Processor-Feature-Register-1?lang=en
> 

-- 
Shaoqin