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SubjectRe: [PATCH v7 10/21] RISC-V: KVM: Handle MMIO exits for VCPU
On Mon, Sep 23, 2019 at 4:42 PM Paolo Bonzini <pbonzini@redhat.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

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