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

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Message-Id: <DA3OJ7WWUGLT.35AVP0QQDJRZV@ventanamicro.com>
Date: Fri, 23 May 2025 18:27:07 +0200
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: 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>, "Atish Patra" <atishp@...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

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.

> 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. :])

Thanks.