linux-kernel - Re: [RFC PATCH 04/16] RISC-V: KVM: Implement VCPU create, init and destroy functions

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Message-ID: <CAAhSdy0OH9h-R=2NxhhPs6jmFPNgZVSwFtCjtJrf++htu82ifA@mail.gmail.com>
Date:   Tue, 30 Jul 2019 17:15:53 +0530
From:   Anup Patel <anup@...infault.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Radim K <rkrcmar@...hat.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Atish Patra <Atish.Patra@....com>,
        Alistair Francis <Alistair.Francis@....com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Christoph Hellwig <hch@...radead.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 04/16] RISC-V: KVM: Implement VCPU create, init and
 destroy functions

On Tue, Jul 30, 2019 at 3:46 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 30/07/19 10:48, Paolo Bonzini wrote:
> > On 29/07/19 13:56, Anup Patel wrote:
> >> +    cntx->hstatus |= HSTATUS_SP2V;
> >> +    cntx->hstatus |= HSTATUS_SP2P;
> > IIUC, cntx->hstatus's SP2P bit contains the guest's sstatus.SPP bit?
>
> Nevermind, that was also a bit confused.  The guest's sstatus.SPP is in
> vsstatus.  The pseudocode for V-mode switch is
>
> SRET:
>   V = hstatus.SPV (1)
>   MODE = sstatus.SPP
>   hstatus.SPV = hstatus.SP2V
>   sstatus.SPP = hstatus.SP2P
>   hstatus.SP2V = 0
>   hstatus.SP2P = 0
>   ...
>
> trap:
>   hstatus.SP2V = hstatus.SPV
>   hstatus.SP2P = sstatus.SPP
>   hstatus.SPV = V (1)
>   sstatus.SPP = MODE
>   V = 0
>   MODE = 1
>

Yes, this kind of pseudo-code are not explicitly specified in the
RISC-V spec. The RISC-V formal model is supposed to cover
this kind of detailed HW state transition.

> so:
>
> 1) indeed we need SP2V=SPV=1 when entering guest mode
>
> 2) sstatus.SPP contains the guest mode
>
> 3) SP2P doesn't really matter for KVM since it never goes to VS-mode
> from an interrupt handler, so if my reasoning is correct I'd leave it
> clear, but I guess it's up to you whether to set it or not.

Yes, SP2P does not matter but we set it to 1 here so that from Guest
perspective it seems we were in S-mode previously.

Regards,
Anup