linux-kernel - Re: [RFC PATCH v2 08/19] RISC-V: KVM: Implement VCPU world-switch

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Date:   Fri, 2 Aug 2019 14:13:52 +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 v2 08/19] RISC-V: KVM: Implement VCPU world-switch

On Fri, Aug 2, 2019 at 2:00 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 02/08/19 09:47, Anup Patel wrote:
> > +     /* Save Host SSTATUS, HSTATUS, SCRATCH and STVEC */
> > +     csrr    t0, CSR_SSTATUS
> > +     REG_S   t0, (KVM_ARCH_HOST_SSTATUS)(a0)
> > +     csrr    t1, CSR_HSTATUS
> > +     REG_S   t1, (KVM_ARCH_HOST_HSTATUS)(a0)
> > +     csrr    t2, CSR_SSCRATCH
> > +     REG_S   t2, (KVM_ARCH_HOST_SSCRATCH)(a0)
> > +     csrr    t3, CSR_STVEC
> > +     REG_S   t3, (KVM_ARCH_HOST_STVEC)(a0)
> > +
>
> A possible optimization: if these cannot change while Linux runs (I am
> thinking especially of STVEC and HSTATUS, but perhaps SSCRATCH can be
> saved on kvm_arch_vcpu_load too) you can avoid the csrr and store.

Actual exception vector of Host Linux is different so we switch STVEC
every time.

HSTATUS.SPV is set whenever we come back from Guest world so
while we are in in-kernel run loop with interrupts enabled we can get
external interrupt and HSTATUS.SPV bit can affect SRET of interrupt
handler. To handle this we switch HSTATUS every time.

The world switch code uses SSCRATCH to save vcpu->arch pointer
which is later used on return path. Now, I did not want to restrict Host
Linux from using SSCRATCH for some other purpose hence we
switch SSCRATCH every time.

Regards,
Anup