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

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Date:   Tue, 30 Jul 2019 12:16:57 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Radim K <rkrcmar@...hat.com>
Cc:     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>,
        Anup Patel <anup@...infault.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 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

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.

Paolo

> I suggest adding a comment here, and again providing a ONE_REG interface
> to sstatus so that the ABI is final before RISC-V KVM is merged.
> 
> What happens if the guest executes SRET?  Is that EXC_SYSCALL in hedeleg?
> 
> (BTW the name of SP2V and SP2P is horrible, I think HPV/HPP or HSPV/HSPP
> would have been clearer, but that's not your fault).