linux-kernel - Re: [PATCH v8 13/14] RISC-V: KVM: add support for FWFT SBI extension

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Message-ID: <5dd587b3-8c04-41d1-b677-5b07266cfec5@linux.dev>
Date: Fri, 23 May 2025 11:02:11 -0700
From: Atish Patra <atish.patra@...ux.dev>
To: Radim Krčmář <rkrcmar@...tanamicro.com>,
 Clément Léger <cleger@...osinc.com>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
 <palmer@...belt.com>, Anup Patel <anup@...infault.org>,
 Atish Patra <atishp@...shpatra.org>, Shuah Khan <shuah@...nel.org>,
 Jonathan Corbet <corbet@....net>, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
 linux-kselftest@...r.kernel.org
Cc: Samuel Holland <samuel.holland@...ive.com>,
 Andrew Jones <ajones@...tanamicro.com>, Deepak Gupta <debug@...osinc.com>,
 Charlie Jenkins <charlie@...osinc.com>,
 linux-riscv <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH v8 13/14] RISC-V: KVM: add support for FWFT SBI extension

On 5/23/25 9:27 AM, Radim KrÄmÃ¡Å wrote:
> 2025-05-23T17:29:49+02:00, Clément Léger <cleger@...osinc.com>:
>> On 23/05/2025 15:05, Radim Krčmář wrote:
>>> 2025-05-23T12:19:30+02:00, Clément Léger <cleger@...osinc.com>:
>>>> +++ b/arch/riscv/kvm/vcpu_sbi_fwft.c
>>>> +static const enum sbi_fwft_feature_t kvm_fwft_defined_features[] = {
>>>> +	SBI_FWFT_MISALIGNED_EXC_DELEG,
>>>> +	SBI_FWFT_LANDING_PAD,
>>>> +	SBI_FWFT_SHADOW_STACK,
>>>> +	SBI_FWFT_DOUBLE_TRAP,
>>>> +	SBI_FWFT_PTE_AD_HW_UPDATING,
>>>> +	SBI_FWFT_POINTER_MASKING_PMLEN,
>>>> +};
>>>
>>> How will userspace control which subset of these features is allowed in
>>> the guest?
>>>
>>> (We can reuse the KVM SBI extension interface if we don't want to add a
>>>   FWFT specific ONE_REG.)
>>
>> Hi Radim,
>>
>> I didn't looked at that part. But most likely using the kvm one reg
>> interface seems ok like what is done for STA ? We could have per feature
>> override with one reg per feature.
> 
> Sounds fine.
> 

Yeah. We can have a follow up series for SBI FWFT state that allows user 
space to toggle each state individually.

>> Is this something blocking though ? We'd like to merge FWFT once SBI 3.0
>> is ratified so that would be nice not delaying it too much. I'll take a
>> look at it to see if it isn't too long to implement.
> 
> Not blocking, but I would at least default FWFT to disabled, because
> current userspace cannot handle [14/14].  (Well... save/restore was
> probably broken even before, but let's try to not make it worse. :])
> 

User space can not enable or disable misaligned access delegation as 
there is no interface for now rightly pointed by you. I guess supporting 
that would be quicker than fixing the broader guest save/restore 
anyways. Isn't it ?

We can have the patches ready for the next MW for FWFT one reg interface.

> Thanks.
> 
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> linux-riscv@...ts.infradead.org
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