linux-kernel - Re: [PATCH v2 2/2] RISC-V: KVM: add KVM_CAP_RISCV_MP_STATE

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Message-Id: <D9ROL5UEYYHX.1B1FE6LZJ9ESO@ventanamicro.com>
Date: Fri, 09 May 2025 15:57:39 +0200
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Anup Patel" <anup@...infault.org>, "Andrew Jones"
 <ajones@...tanamicro.com>
Cc: <kvm-riscv@...ts.infradead.org>, <kvm@...r.kernel.org>,
 <linux-riscv@...ts.infradead.org>, <linux-kernel@...r.kernel.org>, "Atish
 Patra" <atishp@...shpatra.org>, "Paul Walmsley" <paul.walmsley@...ive.com>,
 "Palmer Dabbelt" <palmer@...belt.com>, "Albert Ou" <aou@...s.berkeley.edu>,
 "Alexandre Ghiti" <alex@...ti.fr>
Subject: Re: [PATCH v2 2/2] RISC-V: KVM: add KVM_CAP_RISCV_MP_STATE_RESET

2025-05-09T17:59:28+05:30, Anup Patel <anup@...infault.org>:
> On Fri, May 9, 2025 at 5:49 PM Andrew Jones <ajones@...tanamicro.com> wrote:
>> On Fri, May 09, 2025 at 05:33:49PM +0530, Anup Patel wrote:
>> > On Fri, May 9, 2025 at 2:16 PM Radim Krčmář <rkrcmar@...tanamicro.com> wrote:
>> > > 2025-05-09T12:25:24+05:30, Anup Patel <anup@...infault.org>:
>> > > > On Thu, May 8, 2025 at 8:01 PM Radim Krčmář <rkrcmar@...tanamicro.com> wrote:
>> > > >>  * Preserve the userspace initialized VCPU state on sbi_hart_start.
>> > > >>  * Return to userspace on sbi_hart_stop.
>> > > >
>> > > > There is no userspace involvement required when a Guest VCPU
>> > > > stops itself using SBI HSM stop() call so STRONG NO to this change.
>> > >
>> > > Ok, I'll drop it from v3 -- it can be handled by future patches that
>> > > trap SBI calls to userspace.
>> > >
>> > > The lack of userspace involvement is the issue.  KVM doesn't know what
>> > > the initial state should be.
>> >
>> > The SBI HSM virtualization does not need any KVM userspace
>> > involvement.
>> >
>> > When a VCPU stops itself using SBI HSM stop(), the Guest itself
>> > provides the entry address and argument when starting the VCPU
>> > using SBI HSM start() without any KVM userspace involvement.
>> >
>> > In fact, even at Guest boot time all non-boot VCPUs are brought-up
>> > using SBI HSM start() by the boot VCPU where the Guest itself
>> > provides entry address and argument without any KVM userspace
>> > involvement.
>>
>> HSM only specifies the state of a few registers and the ISA only a few
>> more. All other registers have IMPDEF reset values. Zeros, like KVM
>> selects, are a good choice and the best default, but if the VMM disagrees,
>> then it should be allowed to select what it likes, as the VMM/user is the
>> policy maker and KVM is "just" the accelerator.
>
> Till now there are no such IMPDEF reset values expected. We will
> cross that bridge when needed. Although, I doubt we will ever need it.

The IMPDEF issue already exists.  KVM resets scounteren to 7, but
userspace wants it to be different, likely 0.