linux-kernel - Re: [PATCH 5/5] riscv/efi_stub: Support for 64bit boot-hartid

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Message-ID: <CAOnJCUKbT88c+_PzEuchEtzjEtiHcMFvRH_G98z1AmpSdRH+BA@mail.gmail.com>
Date:   Wed, 25 May 2022 16:49:46 -0700
From:   Atish Patra <atishp@...shpatra.org>
To:     Jessica Clarke <jrtc27@...c27.com>
Cc:     Heinrich Schuchardt <heinrich.schuchardt@...onical.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marc Zyngier <maz@...nel.org>,
        Atish Patra <atishp@...osinc.com>,
        Anup Patel <apatel@...tanamicro.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Sunil V L <sunil.vl@...il.com>,
        Sunil V L <sunilvl@...tanamicro.com>
Subject: Re: [PATCH 5/5] riscv/efi_stub: Support for 64bit boot-hartid

On Wed, May 25, 2022 at 4:36 PM Jessica Clarke <jrtc27@...c27.com> wrote:
>
> On 26 May 2022, at 00:11, Atish Patra <atishp@...shpatra.org> wrote:
> >
> > On Wed, May 25, 2022 at 9:09 AM Heinrich Schuchardt
> > <heinrich.schuchardt@...onical.com> wrote:
> >>
> >> On 5/25/22 17:48, Ard Biesheuvel wrote:
> >>> On Wed, 25 May 2022 at 17:11, Sunil V L <sunilvl@...tanamicro.com> wrote:
> >>>>
> >>>> 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.
> >>>>
> >>>> Signed-off-by: Sunil V L <sunilvl@...tanamicro.com>
> >>>> ---
> >>>> drivers/firmware/efi/libstub/riscv-stub.c | 12 +++++++++---
> >>>> 1 file changed, 9 insertions(+), 3 deletions(-)
> >>>>
> >>>> 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;
> >>>>
> >>>> fdt = get_efi_config_table(DEVICE_TREE_GUID);
> >>>> if (!fdt)
> >>>> @@ -40,10 +40,16 @@ static int get_boot_hartid_from_fdt(void)
> >>>> return -EINVAL;
> >>>>
> >>>> prop = fdt_getprop((void *)fdt, chosen_node, "boot-hartid", &len);
> >>>> - if (!prop || len != sizeof(u32))
> >>>> + if (!prop)
> >>>> + return -EINVAL;
> >>>> +
> >>>> + if (len == sizeof(u32))
> >>>> + hartid = (unsigned long) fdt32_to_cpu(*(fdt32_t *)prop);
> >>>> + else if (len == sizeof(u64))
> >>>> + hartid = (unsigned long) fdt64_to_cpu(*(fdt64_t *)prop);
> >>>
> >>> 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.
> >>
> >> When running in S-mode the SBI handles unaligned access but this has a
> >> performance penalty.
> >>
> >> We could use fdt64_to_cpu(__get_unaligned_t(fdt64_t, prop)) here.
> >>
> >
> > 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?
>
To support prctl call with PR_SET_UNALIGN

> 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.
> >
> >> Best regards
> >>
> >> Heinrich
> >>
> >>>
> >>>
> >>>> + else
> >>>> return -EINVAL;
> >>>>
> >>>> - hartid = fdt32_to_cpu(*prop);
> >>>> return 0;
> >>>> }
> >>>>
> >>>> --
> >>>> 2.25.1
> >>>>
> >>
> >
> >
> > --
> > Regards,
> > Atish
> >
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv@...ts.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv
>


-- 
Regards,
Atish

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