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

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Message-Id: <DA5YVKW682V3.2DODRY4EDL3IW@ventanamicro.com>
Date: Mon, 26 May 2025 10:58:46 +0200
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Atish Patra" <atish.patra@...ux.dev>,
 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

2025-05-23T11:02:11-07:00, Atish Patra <atish.patra@...ux.dev>:
> On 5/23/25 9:27 AM, Radim KrÄmÃ¡Å wrote:
>> 2025-05-23T17:29:49+02:00, Clément Léger <cleger@...osinc.com>:
>>> 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 mean setting default_disabled=true and just disabling FWFT for the
guest unless userspace explicitly enables the incomplete extension.
We would blame the user for wanting mutually exclusive features.

>                                                       I guess supporting 
> that would be quicker than fixing the broader guest save/restore 
> anyways. Isn't it ?

Yes.  The save/restore for FWFT is simple (if we disregard the
discussions), but definitely more than a single line.