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

From: "Clément Léger" <cleger@rivosinc.com>
To: "Atish Patra" <atish.patra@linux.dev>,
	"Radim Krčmář" <rkrcmar@ventanamicro.com>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Anup Patel" <anup@brainfault.org>,
	"Atish Patra" <atishp@atishpatra.org>,
	"Shuah Khan" <shuah@kernel.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-doc@vger.kernel.org, kvm@vger.kernel.org,
	kvm-riscv@lists.infradead.org, linux-kselftest@vger.kernel.org
Cc: Samuel Holland <samuel.holland@sifive.com>,
	Andrew Jones <ajones@ventanamicro.com>,
	Deepak Gupta <debug@rivosinc.com>,
	Charlie Jenkins <charlie@rivosinc.com>,
	linux-riscv <linux-riscv-bounces@lists.infradead.org>
Subject: Re: [PATCH v8 13/14] RISC-V: KVM: add support for FWFT SBI extension
Date: Fri, 23 May 2025 21:23:41 +0200	[thread overview]
Message-ID: <01696003-e2b4-4450-85cf-715e31b136d5@rivosinc.com> (raw)
In-Reply-To: <5dd587b3-8c04-41d1-b677-5b07266cfec5@linux.dev>

On 23/05/2025 20:02, Atish Patra wrote:
> On 5/23/25 9:27 AM, Radim KrÄmÃ¡Å wrote:
>> 2025-05-23T17:29:49+02:00, Clément Léger <cleger@rivosinc.com>:
>>> On 23/05/2025 15:05, Radim Krčmář wrote:
>>>> 2025-05-23T12:19:30+02:00, Clément Léger <cleger@rivosinc.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.

Yeah sure I'll work on that in the meantime.

> 
>> Thanks.
>>
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> 