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

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Date:   Mon, 5 Aug 2019 13:37:50 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Anup Patel <Anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Paolo Bonzini <pbonzini@...hat.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 02.08.19 09:47, Anup Patel wrote:
> For KVM RISC-V, we use KVM_GET_ONE_REG/KVM_SET_ONE_REG ioctls to access
> VCPU config and registers from user-space.
> 
> We have three types of VCPU registers:
> 1. CONFIG - these are VCPU config and capabilities
> 2. CORE   - these are VCPU general purpose registers
> 3. CSR    - these are VCPU control and status registers
> 
> The CONFIG registers available to user-space are ISA and TIMEBASE. Out
> of these, TIMEBASE is a read-only register which inform user-space about
> VCPU timer base frequency. The ISA register is a read and write register
> where user-space can only write the desired VCPU ISA capabilities before
> running the VCPU.
> 
> The CORE registers available to user-space are PC, RA, SP, GP, TP, A0-A7,
> T0-T6, S0-S11 and MODE. Most of these are RISC-V general registers except
> PC and MODE. The PC register represents program counter whereas the MODE
> register represent VCPU privilege mode (i.e. S/U-mode).
> 
> The CSRs available to user-space are SSTATUS, SIE, STVEC, SSCRATCH, SEPC,
> SCAUSE, STVAL, SIP, and SATP. All of these are read/write registers.
> 
> In future, more VCPU register types will be added (such as FP) for the
> KVM_GET_ONE_REG/KVM_SET_ONE_REG ioctls.

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.