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SubjectRe: [PATCH v7 10/21] RISC-V: KVM: Handle MMIO exits for VCPU
From
On Mon, 23 Sep 2019 04:12:17 PDT (-0700), 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.
>
> 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?
>
>> + 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. :)
>
> Paolo
>
>> + "li %[tscause], 0\n"
>> +#ifdef CONFIG_64BIT
>> + "ld %[val], (%[addr])\n"
>> +#else
>> + "lw %[val], (%[addr])\n"
>> +#endif
To: anup@brainfault.org
CC: pbonzini@redhat.com
CC: Anup Patel <Anup.Patel@wdc.com>
CC: Paul Walmsley <paul.walmsley@sifive.com>
CC: rkrcmar@redhat.com
CC: daniel.lezcano@linaro.org
CC: tglx@linutronix.de
CC: graf@amazon.com
CC: Atish Patra <Atish.Patra@wdc.com>
CC: Alistair Francis <Alistair.Francis@wdc.com>
CC: Damien Le Moal <Damien.LeMoal@wdc.com>
CC: Christoph Hellwig <hch@infradead.org>
CC: kvm@vger.kernel.org
CC: linux-riscv@lists.infradead.org
CC: linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 10/21] RISC-V: KVM: Handle MMIO exits for VCPU
In-Reply-To: <CAAhSdy1-1yxMnjzppmUBxtSOAuwWaPtNZwW+QH1O7LAnEVP8pg@mail.gmail.com>

On Mon, 23 Sep 2019 06:09:43 PDT (-0700), anup@brainfault.org wrote:
> 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
To: pbonzini@redhat.com
CC: anup@brainfault.org
CC: Anup Patel <Anup.Patel@wdc.com>
CC: Paul Walmsley <paul.walmsley@sifive.com>
CC: rkrcmar@redhat.com
CC: daniel.lezcano@linaro.org
CC: tglx@linutronix.de
CC: graf@amazon.com
CC: Atish Patra <Atish.Patra@wdc.com>
CC: Alistair Francis <Alistair.Francis@wdc.com>
CC: Damien Le Moal <Damien.LeMoal@wdc.com>
CC: Christoph Hellwig <hch@infradead.org>
CC: kvm@vger.kernel.org
CC: linux-riscv@lists.infradead.org
CC: linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 10/21] RISC-V: KVM: Handle MMIO exits for VCPU
In-Reply-To: <45fc3ee5-0f68-4e94-cfb3-0727ca52628f@redhat.com>

On Mon, 23 Sep 2019 06:33:14 PDT (-0700), pbonzini@redhat.com wrote:
> On 23/09/19 15:09, Anup Patel wrote:
>>>> +#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.
>
> ".option norvc"?
>
> Paolo
To: anup@brainfault.org
CC: pbonzini@redhat.com
CC: Anup Patel <Anup.Patel@wdc.com>
CC: Paul Walmsley <paul.walmsley@sifive.com>
CC: rkrcmar@redhat.com
CC: daniel.lezcano@linaro.org
CC: tglx@linutronix.de
CC: graf@amazon.com
CC: Atish Patra <Atish.Patra@wdc.com>
CC: Alistair Francis <Alistair.Francis@wdc.com>
CC: Damien Le Moal <Damien.LeMoal@wdc.com>
CC: Christoph Hellwig <hch@infradead.org>
CC: kvm@vger.kernel.org
CC: linux-riscv@lists.infradead.org
CC: linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 10/21] RISC-V: KVM: Handle MMIO exits for VCPU
In-Reply-To: <CAAhSdy29gi2d9c9tumtO68QbB=_+yUYp+ikN3dQ-wa2e-Lesfw@mail.gmail.com>

On Mon, 23 Sep 2019 22:07:43 PDT (-0700), anup@brainfault.org wrote:
> On Mon, Sep 23, 2019 at 7:03 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> On 23/09/19 15:09, Anup Patel wrote:
>> >>> +#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.
>>
>> ".option norvc"?
>
> Thanks for the hint. I will try ".option norvc"

It should be something like

.option push
.option norvc
ld ...
.option pop

which preserves C support for the rest of the file.

>
> Regards,
> Anup
>
>>
>> Paolo

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