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

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Message-ID: <f9dee99d-f536-e351-f637-b5098d53be22@redhat.com>
Date:   Fri, 2 Aug 2019 10:59:36 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Anup Patel <anup@...infault.org>
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 02/08/19 10:43, Anup Patel wrote:
>> 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.

Right, I'm not saying not to save these registers.  I'm saying not to
read the host value on every world switch, instead load it in
hardware_enable (if it's the same for all physical CPUs) or
kvm_arch_vcpu_load (if it's different for every physical CPU).

IIUC Linux does not use SSCRATCH while in the kernel (it must be zero
while handling an exception, but handle_exception takes care of that).
I think it's okay if you make this assumption, but if you don't want to
make it, you can still save it in kvm_arch_vcpu_load rather than here
since you "own" the thread while in KVM_RUN.

Paolo