linux-kernel - Re: [RFC PATCH 11/16] RISC-V: KVM: Implement stage2 page table programming

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Message-ID: <CAAhSdy09Uhkg=-m213SeR92M1PRx1ZtE-fTLT=nNvg_0HY2YnA@mail.gmail.com>
Date:   Tue, 30 Jul 2019 17:44:03 +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 11/16] RISC-V: KVM: Implement stage2 page table programming

On Tue, Jul 30, 2019 at 2:30 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 29/07/19 13:57, Anup Patel wrote:
> > This patch implements all required functions for programming
> > the stage2 page table for each Guest/VM.
> >
> > At high-level, the flow of stage2 related functions is similar
> > from KVM ARM/ARM64 implementation but the stage2 page table
> > format is quite different for KVM RISC-V.
>
> FWIW I very much prefer KVM x86's recursive implementation of the MMU to
> the hardcoding of pgd/pmd/pte.  I am not asking you to rewrite it, but
> I'll mention it because I noticed that you do not support 48-bit guest
> physical addresses.

Yes, I also prefer recursive page table programming. In fact, the first
hypervisor we ported for RISC-V was Xvisor and over there have
recursive page table programming for both stage1 and stage2.

BTW, 48bit VA and guest physical address is already defined in
latest RISC-V spec. It's just that there is not HW (or QEMU) implementation
as of now for 4-level page table.

I will certainly add this to our TODO list.

Regards,
Anup