Re: [PATCH v5 0/7] arm64: efi: leave MMU and caches on at boot

[Ard Biesheuvel <ardb@kernel.org>]

On Sat, 26 Nov 2022 at 15:16, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Tue, 15 Nov 2022 at 12:31, Will Deacon <will@kernel.org> wrote:
> >
> > On Tue, Nov 15, 2022 at 12:21:55PM +0100, Ard Biesheuvel wrote:
> > > On Tue, 15 Nov 2022 at 12:17, Will Deacon <will@kernel.org> wrote:
> > > >
> > > > On Fri, Nov 11, 2022 at 05:36:19PM +0000, Mark Rutland wrote:
> > > > > On Tue, Nov 08, 2022 at 07:21:57PM +0100, Ard Biesheuvel wrote:
> > > > > > The purpose of this series is to remove any explicit cache maintenance
> > > > > > for coherency during early boot that becomes unnecessary if we simply
> > > > > > retain the cacheable 1:1 mapping of all of system RAM provided by EFI,
> > > > > > and use it to populate the ID map page tables. After setting up this
> > > > > > preliminary ID map, we disable the MMU, drop to EL1, reprogram the MAIR,
> > > > > > TCR and SCTLR registers as before, and proceed as usual, avoiding the
> > > > > > need for any manipulations of memory while the MMU and caches are off.
> > > > > >
> > > > > > The only properties of the firmware provided 1:1 map we rely on is that
> > > > > > it does not require any explicit cache maintenance for coherency, and
> > > > > > that it covers the entire memory footprint of the image, including the
> > > > > > BSS and padding at the end - all else is under control of the kernel
> > > > > > itself, as before.
> > > > >
> > > > > As a high-level thing, I'm still very much not keen on entering the kernel with
> > > > > the MMU on. Given that we have to support booting with the MMU off for !EFI
> > > > > boot (including kexec when EFI is in use), I think this makes it harder to
> > > > > reason about the boot code overall (e.g. due to the conditional maintenance
> > > > > added to head.S), and adds more scope for error, even if it simplifies the EFI
> > > > > stub itself.
> > > >
> > > > As discussed offline, two things that would help the current series are:
> > > >
> > > >   (1) Some performance numbers comparing MMU off vs MMU on boot
> > > >
>
> Finally got around to measuring this - I lost access to my TX2 machine
> for a couple of days during the past week,
>
> With the patch below applied to mainline, I measure ~6 ms spent
> cleaning the entire image to the PoC (which is the bulk of it) and
> subsequently populating the initial ID map and activating it.
>
> This drops to about 0.6 ms with my changes applied. This is unlikely
> to ever matter in practice, perhaps, but I will note that booting a VM
> in EFI mode using Tianocore/EDK2 from the point where KVM clears the
> counter to the point where we start user space can be done (on the
> same machine) in 500-700 ms so it is not entirely insignificant
> either.
>
> I could try and measure it on bare metal as well, but I suppose that
> launch times are even less relevant there so I didn't bother.

diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
index 61a87fa1c3055e26..27f59784a1c0be2c 100644
--- a/arch/arm64/kernel/efi-entry.S
+++ b/arch/arm64/kernel/efi-entry.S
@@ -22,6 +22,7 @@ SYM_CODE_START(efi_enter_kernel)
        ldr     w2, =primary_entry_offset
        add     x19, x0, x2             // relocated Image entrypoint
        mov     x20, x1                 // DTB address
+       mrs     x27, cntvct_el0

        /*
         * Clean the copied Image to the PoC, and ensure it is not shadowed by
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 2196aad7b55bcef0..068a7d111836382b 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -430,6 +430,8 @@ SYM_FUNC_START_LOCAL(__primary_switched)

        str_l   x21, __fdt_pointer, x5          // Save FDT pointer

+       str_l   x27, boot_args + 8, x5
+
        ldr_l   x4, kimage_vaddr                // Save the offset between
        sub     x4, x4, x0                      // the kernel virtual and
        str_l   x4, kimage_voffset, x5          // physical mappings
@@ -797,6 +799,10 @@ SYM_FUNC_START_LOCAL(__primary_switch)
        adrp    x1, reserved_pg_dir
        adrp    x2, init_idmap_pg_dir
        bl      __enable_mmu
+
+       mrs     x0, cntvct_el0
+       sub     x27, x0, x27
+
 #ifdef CONFIG_RELOCATABLE
        adrp    x23, KERNEL_START
        and     x23, x23, MIN_KIMG_ALIGN - 1