linux-kernel - Re: [PATCH v7 10/21] RISC-V: KVM: Handle MMIO exits for VCPU

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Date:   Mon, 23 Sep 2019 18:39:43 +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>,
        Alexander Graf <graf@...zon.com>,
        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: [PATCH v7 10/21] RISC-V: KVM: Handle MMIO exits for VCPU

On Mon, Sep 23, 2019 at 4:42 PM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 04/09/19 18:15, Anup Patel wrote:
> > +     unsigned long guest_sstatus =
> > +                     vcpu->arch.guest_context.sstatus | SR_MXR;
> > +     unsigned long guest_hstatus =
> > +                     vcpu->arch.guest_context.hstatus | HSTATUS_SPRV;
> > +     unsigned long guest_vsstatus, old_stvec, tmp;
> > +
> > +     guest_sstatus = csr_swap(CSR_SSTATUS, guest_sstatus);
> > +     old_stvec = csr_swap(CSR_STVEC, (ulong)&__kvm_riscv_unpriv_trap);
> > +
> > +     if (read_insn) {
> > +             guest_vsstatus = csr_read_set(CSR_VSSTATUS, SR_MXR);
>
> Is this needed?  IIUC SSTATUS.MXR encompasses a wider set of permissions:
>
>   The HS-level MXR bit makes any executable page readable.  {\tt
>   vsstatus}.MXR makes readable those pages marked executable at the VS
>   translation level, but only if readable at the guest-physical
>   translation level.
>
> So it should be enough to set SSTATUS.MXR=1 I think.  But you also
> shouldn't set SSTATUS.MXR=1 in the !read_insn case.

I was being overly cautious here. Initially, I thought SSTATUS.MXR
applies only to Stage2 and VSSTATUS.MXR applies only to Stage1.

I agree with you. The HS-mode should only need to set SSTATUS.MXR.

>
> Also, you can drop the irq save/restore (which is already a save/restore
> of SSTATUS) since you already write 0 to SSTATUS.SIE in your csr_swap.
> Perhaps add a BUG_ON(guest_sstatus & SR_SIE) before the csr_swap?

I had already dropped irq save/restore in v7 series and having BUG_ON()
on guest_sstatus here would be better.

>
> > +             asm volatile ("\n"
> > +                     "csrrw %[hstatus], " STR(CSR_HSTATUS) ", %[hstatus]\n"
> > +                     "li %[tilen], 4\n"
> > +                     "li %[tscause], 0\n"
> > +                     "lhu %[val], (%[addr])\n"
> > +                     "andi %[tmp], %[val], 3\n"
> > +                     "addi %[tmp], %[tmp], -3\n"
> > +                     "bne %[tmp], zero, 2f\n"
> > +                     "lhu %[tmp], 2(%[addr])\n"
> > +                     "sll %[tmp], %[tmp], 16\n"
> > +                     "add %[val], %[val], %[tmp]\n"
> > +                     "2: csrw " STR(CSR_HSTATUS) ", %[hstatus]"
> > +             : [hstatus] "+&r"(guest_hstatus), [val] "=&r" (val),
> > +               [tmp] "=&r" (tmp), [tilen] "+&r" (tilen),
> > +               [tscause] "+&r" (tscause)
> > +             : [addr] "r" (addr));
> > +             csr_write(CSR_VSSTATUS, guest_vsstatus);
>
> >
> > +#ifndef CONFIG_RISCV_ISA_C
> > +                     "li %[tilen], 4\n"
> > +#else
> > +                     "li %[tilen], 2\n"
> > +#endif
>
> Can you use an assembler directive to force using a non-compressed
> format for ld and lw?  This would get rid of tilen, which is costing 6
> bytes (if I did the RVC math right) in order to save two. :)

I tried looking for it but could not find any assembler directive
to selectively turn-off instruction compression.

>
> Paolo
>
> > +                     "li %[tscause], 0\n"
> > +#ifdef CONFIG_64BIT
> > +                     "ld %[val], (%[addr])\n"
> > +#else
> > +                     "lw %[val], (%[addr])\n"
> > +#endif

Regards,
Anup