linux-kernel - Re: [RFC PATCH v2 07/19] RISC-V: KVM: Implement KVM_GET_ONE_REG/KVM_SET_ONE

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Message-ID: <8563b869-1ab4-d0f1-afad-9cd864b6019a@redhat.com>
Date:   Mon, 5 Aug 2019 13:56:34 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        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 v2 07/19] RISC-V: KVM: Implement
 KVM_GET_ONE_REG/KVM_SET_ONE_REG ioctls

On 05/08/19 13:37, Christian Borntraeger wrote:
> While have ONE_REG will certainly work, have you considered the sync_reg scheme
> (registers as part of kvm_run structure)
> This will speed up the exit to QEMU as you do not have to do multiple ioctls
> (each imposing a systemcall overhead) for one exit. 
> 
> Ideally you should not exit too often into qemu, but for those cases sync_regs
> is faster than ONE_REG.

At least in theory, RISC-V should never have exits to QEMU that need
accessing the registers.  (The same is true for x86; Google implemented
sync_regs because they moved the instruction emulator to userspace in
their fork).

Paolo