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Subject: Re: [PATCH 5/5] riscv/efi_stub: Support for 64bit boot-hartid
From:   Jessica Clarke <jrtc27@jrtc27.com>
In-Reply-To: <CAOnJCU+r6KgR7bd2dx5QLmmVLjJX8GhETHb6rG65wq0e_m6FVA@mail.gmail.com>
Date:   Thu, 26 May 2022 00:36:24 +0100
Cc:     Heinrich Schuchardt <heinrich.schuchardt@canonical.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Marc Zyngier <maz@kernel.org>,
        Atish Patra <atishp@rivosinc.com>,
        Anup Patel <apatel@ventanamicro.com>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-efi <linux-efi@vger.kernel.org>,
        Sunil V L <sunil.vl@gmail.com>,
        Sunil V L <sunilvl@ventanamicro.com>
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References: <20220525151106.2176147-1-sunilvl@ventanamicro.com>
 <20220525151106.2176147-6-sunilvl@ventanamicro.com>
 <CAMj1kXFhEBv7MVCKZuXdx9=hZx3qWbkATdLDwXAe_Zn9Xyx=dg@mail.gmail.com>
 <1e90b15b-8c73-0de8-2885-1292923b7575@canonical.com>
 <CAOnJCU+r6KgR7bd2dx5QLmmVLjJX8GhETHb6rG65wq0e_m6FVA@mail.gmail.com>
To:     Atish Patra <atishp@atishpatra.org>
Precedence: bulk

On 26 May 2022, at 00:11, Atish Patra <atishp@atishpatra.org> wrote:
>=20
> On Wed, May 25, 2022 at 9:09 AM Heinrich Schuchardt
> <heinrich.schuchardt@canonical.com> wrote:
>>=20
>> On 5/25/22 17:48, Ard Biesheuvel wrote:
>>> On Wed, 25 May 2022 at 17:11, Sunil V L <sunilvl@ventanamicro.com> =
wrote:
>>>>=20
>>>> The boot-hartid can be a 64bit value on RV64 platforms. Currently,
>>>> the "boot-hartid" in DT is assumed to be 32bit only. This patch
>>>> detects the size of the "boot-hartid" and uses 32bit or 64bit
>>>> FDT reads appropriately.
>>>>=20
>>>> Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
>>>> ---
>>>> drivers/firmware/efi/libstub/riscv-stub.c | 12 +++++++++---
>>>> 1 file changed, 9 insertions(+), 3 deletions(-)
>>>>=20
>>>> diff --git a/drivers/firmware/efi/libstub/riscv-stub.c =
b/drivers/firmware/efi/libstub/riscv-stub.c
>>>> index 9e85e58d1f27..d748533f1329 100644
>>>> --- a/drivers/firmware/efi/libstub/riscv-stub.c
>>>> +++ b/drivers/firmware/efi/libstub/riscv-stub.c
>>>> @@ -29,7 +29,7 @@ static int get_boot_hartid_from_fdt(void)
>>>> {
>>>> const void *fdt;
>>>> int chosen_node, len;
>>>> - const fdt32_t *prop;
>>>> + const void *prop;
>>>>=20
>>>> fdt =3D get_efi_config_table(DEVICE_TREE_GUID);
>>>> if (!fdt)
>>>> @@ -40,10 +40,16 @@ static int get_boot_hartid_from_fdt(void)
>>>> return -EINVAL;
>>>>=20
>>>> prop =3D fdt_getprop((void *)fdt, chosen_node, "boot-hartid", =
&len);
>>>> - if (!prop || len !=3D sizeof(u32))
>>>> + if (!prop)
>>>> + return -EINVAL;
>>>> +
>>>> + if (len =3D=3D sizeof(u32))
>>>> + hartid =3D (unsigned long) fdt32_to_cpu(*(fdt32_t *)prop);
>>>> + else if (len =3D=3D sizeof(u64))
>>>> + hartid =3D (unsigned long) fdt64_to_cpu(*(fdt64_t *)prop);
>>>=20
>>> Does RISC-V care about alignment? A 64-bit quantity is not =
guaranteed
>>> to appear 64-bit aligned in the DT, and the cast violates C =
alignment
>>> rules, so this should probably used get_unaligned_be64() or =
something
>>> like that.
>>=20
>> When running in S-mode the SBI handles unaligned access but this has =
a
>> performance penalty.
>>=20
>> We could use fdt64_to_cpu(__get_unaligned_t(fdt64_t, prop)) here.
>>=20
>=20
> It is better to avoid unaligned access in the kernel. There are some
> plans to disable
> misaligned load/store emulation in the firmware if user space requests
> it via prctl.

Why?

Jess

> We need another SBI extension to do that. The idea is to keep it
> enabled by default in the firmware but
> userspace should have an option to disable it via prctl. If we make
> sure that the kernel doesn't invoke any
> unaligned access, this feature can be implemented easily.
>=20
>> Best regards
>>=20
>> Heinrich
>>=20
>>>=20
>>>=20
>>>> + else
>>>> return -EINVAL;
>>>>=20
>>>> - hartid =3D fdt32_to_cpu(*prop);
>>>> return 0;
>>>> }
>>>>=20
>>>> --
>>>> 2.25.1
>>>>=20
>>=20
>=20
>=20
> --=20
> Regards,
> Atish
>=20
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