Re: [PATCH v3 2/2] ARM: support PHYS_OFFSET minimum aligned at 64KiB boundary

From: Ard Biesheuvel
Date: Thu Sep 17 2020 - 02:55:06 EST

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In reply to: Leizhen (ThunderTown): "Re: [PATCH v3 2/2] ARM: support PHYS_OFFSET minimum aligned at 64KiB boundary"
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On Thu, 17 Sep 2020 at 05:16, Leizhen (ThunderTown)
<thunder.leizhen@xxxxxxxxxx> wrote:
>
>
>
> On 2020/9/16 19:15, Ard Biesheuvel wrote:
> > (+ Arnd, Nico)
> >
> > On Wed, 16 Sep 2020 at 05:51, Zhen Lei <thunder.leizhen@xxxxxxxxxx> wrote:
> >>
> >> Currently, only support the kernels where the base of physical memory is
> >> at a 16MiB boundary. Because the add/sub instructions only contains 8bits
> >> unrotated value. But we can use one more "add/sub" instructions to handle
> >> bits 23-16. The performance will be slightly affected.
> >>
> >> Since most boards meet 16 MiB alignment, so add a new configuration
> >> option ARM_PATCH_PHYS_VIRT_RADICAL (default n) to control it. Say Y if
> >> anyone really needs it.
> >>
> >> All r0-r7 (r1 = machine no, r2 = atags or dtb, in the start-up phase) are
> >> used in __fixup_a_pv_table() now, but the callee saved r11 is not used in
> >> the whole head.S file. So choose it.
> >>
> >> Because the calculation of "y = x + __pv_offset[63:24]" have been done,
> >> so we only need to calculate "y = y + __pv_offset[23:16]", that's why
> >> the parameters "to" and "from" of __pv_stub() and __pv_add_carry_stub()
> >> in the scope of CONFIG_ARM_PATCH_PHYS_VIRT_RADICAL are all passed "t"
> >> (above y).
> >>
> >> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
> >> ---
> >> arch/arm/Kconfig | 17 ++++++++++++++++-
> >> arch/arm/include/asm/memory.h | 16 +++++++++++++---
> >> arch/arm/kernel/head.S | 25 +++++++++++++++++++------
> >> 3 files changed, 48 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> >> index e00d94b16658765..073dafa428f3c87 100644
> >> --- a/arch/arm/Kconfig
> >> +++ b/arch/arm/Kconfig
> >> @@ -240,12 +240,27 @@ config ARM_PATCH_PHYS_VIRT
> >> kernel in system memory.
> >>
> >> This can only be used with non-XIP MMU kernels where the base
> >> - of physical memory is at a 16MB boundary.
> >> + of physical memory is at a 16MiB boundary.
> >>
> >> Only disable this option if you know that you do not require
> >> this feature (eg, building a kernel for a single machine) and
> >> you need to shrink the kernel to the minimal size.
> >>
> >> +config ARM_PATCH_PHYS_VIRT_RADICAL
> >> + bool "Support PHYS_OFFSET minimum aligned at 64KiB boundary"
> >> + depends on ARM_PATCH_PHYS_VIRT
> >> + depends on !THUMB2_KERNEL
> >
> > Why is this not implemented for Thumb2 too?
>
> No Thumb2 boards.
>

Are you saying none of the boards you tested this on have support for
the Thumb2 instruction set?? That seems unlikely to me, given that all
ARM cores since Cortex-A8 can run this code, but even if that were
true, you can easily test this on QEMU as well, which can definitely
run the Thumb2 kernel.

> >
> > Also, as Russell points out as well, this may end up being enabled for
> > all multiarch kernels, so it makes sense to explore whether we can
> > enable this unconditionally.
>
> Yes, In fact, I think we can consider enabling this unconditionally after
> the THUMB2 branch is implemented. Performance and code size should not be
> a problem.
>

Excellent.

> > Do you have any numbers wrt the impact on
> > text size? I would assume it is negligible, but numbers help.
>
> The text size increased a bit more than 2 KB (2164 Bytes), about 0.0146%.
>
> make ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- distclean defconfig
>
> Before:
> $ size vmlinux
> text data bss dec hex filename
> 14781964 7508366 420080 22710410 15a888a vmlinux
>
> After:
> $ size vmlinux
> text data bss dec hex filename
> 14784128 7508366 420080 22712574 15a90fe vmlinux
>
>

OK, so negligible in terms of code size.

> >
> > Being able to decompress the image to any 2MiB aligned base address is
> > also quite useful for EFI boot, and it may also help to get rid of the
> > TEXT_OFFSET hacks we have for some platforms in the future.>
> >
> >> + help
> >> + This can only be used with non-XIP MMU kernels where the base
> >> + of physical memory is at a 64KiB boundary.
> >> +
> >> + Compared with ARM_PATCH_PHYS_VIRT, one or two more instructions
> >> + need to be added to implement the conversion of bits 23-16 of
> >> + the VA/PA in phys-to-virt and virt-to-phys. The performance is
> >> + slightly affected.
> >> +
> >
> > Does it affect performance in other ways beyond code size/Icache density?
>
> I just want to say it will slightly slower than !ARM_PATCH_PHYS_VIRT_RADICAL,
> because one or two more instructions. It certainly cannot affect system performance.
>
> Because of your doubts, I think I should remove the statement: "The performance is
> slightly affected."
>

Yeah, I think you can drop that statement.

> >
> >> + If unsure say N here.
> >> +
> >> config NEED_MACH_IO_H
> >> bool
> >> help
> >> diff --git a/arch/arm/include/asm/memory.h b/arch/arm/include/asm/memory.h
> >> index 99035b5891ef442..f97b37303a00f60 100644
> >> --- a/arch/arm/include/asm/memory.h
> >> +++ b/arch/arm/include/asm/memory.h
> >> @@ -173,6 +173,7 @@
> >> * so that all we need to do is modify the 8-bit constant field.
> >> */
> >> #define __PV_BITS_31_24 0x81000000
> >> +#define __PV_BITS_23_16 0x00810000
> >> #define __PV_BITS_7_0 0x81
> >>
> >> extern unsigned long __pv_phys_pfn_offset;
> >> @@ -201,7 +202,7 @@
> >> : "=r" (t) \
> >> : "I" (__PV_BITS_7_0))
> >>
> >> -#define __pv_add_carry_stub(x, y) \
> >> +#define __pv_add_carry_stub(x, y, type) \
> >> __asm__ volatile("@ __pv_add_carry_stub\n" \
> >> "1: adds %Q0, %1, %2\n" \
> >> " adc %R0, %R0, #0\n" \
> >> @@ -209,7 +210,7 @@
> >> " .long 1b\n" \
> >> " .popsection\n" \
> >> : "+r" (y) \
> >> - : "r" (x), "I" (__PV_BITS_31_24) \
> >> + : "r" (x), "I" (type) \
> >> : "cc")
> >>
> >> static inline phys_addr_t __virt_to_phys_nodebug(unsigned long x)
> >> @@ -218,9 +219,15 @@ static inline phys_addr_t __virt_to_phys_nodebug(unsigned long x)
> >>
> >> if (sizeof(phys_addr_t) == 4) {
> >> __pv_stub(x, t, "add", __PV_BITS_31_24);
> >> +#ifdef CONFIG_ARM_PATCH_PHYS_VIRT_RADICAL
> >> + __pv_stub(t, t, "add", __PV_BITS_23_16);
> >> +#endif
> >> } else {
> >> __pv_stub_mov_hi(t);
> >> - __pv_add_carry_stub(x, t);
> >> + __pv_add_carry_stub(x, t, __PV_BITS_31_24);
> >> +#ifdef CONFIG_ARM_PATCH_PHYS_VIRT_RADICAL
> >> + __pv_add_carry_stub(t, t, __PV_BITS_23_16);
> >> +#endif
> >> }
> >> return t;
> >> }
> >> @@ -236,6 +243,9 @@ static inline unsigned long __phys_to_virt(phys_addr_t x)
> >> * in place where 'r' 32 bit operand is expected.
> >> */
> >> __pv_stub((unsigned long) x, t, "sub", __PV_BITS_31_24);
> >> +#ifdef CONFIG_ARM_PATCH_PHYS_VIRT_RADICAL
> >> + __pv_stub(t, t, "sub", __PV_BITS_23_16);
> >> +#endif
> >> return t;
> >> }
> >>
> >> diff --git a/arch/arm/kernel/head.S b/arch/arm/kernel/head.S
> >> index 02d78c9198d0e8d..d9fb226a24d43ae 100644
> >> --- a/arch/arm/kernel/head.S
> >> +++ b/arch/arm/kernel/head.S
> >> @@ -120,7 +120,7 @@ ENTRY(stext)
> >> bl __fixup_smp
> >> #endif
> >> #ifdef CONFIG_ARM_PATCH_PHYS_VIRT
> >> - bl __fixup_pv_table
> >> + bl __fixup_pv_table @r11 will be used
> >> #endif
> >> bl __create_page_tables
> >>
> >> @@ -614,8 +614,13 @@ __fixup_pv_table:
> >> mov r0, r8, lsr #PAGE_SHIFT @ convert to PFN
> >> str r0, [r6] @ save computed PHYS_PFN_OFFSET to __pv_phys_pfn_offset
> >> strcc ip, [r7, #HIGH_OFFSET] @ save to __pv_offset high bits
> >> +#ifdef CONFIG_ARM_PATCH_PHYS_VIRT_RADICAL
> >> + mov r6, r3, lsr #16 @ constant for add/sub instructions
> >> + teq r3, r6, lsl #16 @ must be 64KiB aligned
> >> +#else
> >> mov r6, r3, lsr #24 @ constant for add/sub instructions
> >> teq r3, r6, lsl #24 @ must be 16MiB aligned
> >> +#endif
> >> THUMB( it ne @ cross section branch )
> >> bne __error
> >> str r3, [r7, #LOW_OFFSET] @ save to __pv_offset low bits
> >> @@ -636,7 +641,9 @@ __fixup_a_pv_table:
> >> add r6, r6, r3
> >> ldr r0, [r6, #HIGH_OFFSET] @ __pv_offset high word
> >> ldr r6, [r6, #LOW_OFFSET] @ __pv_offset low word
> >> - mov r6, r6, lsr #24
> >> + mov r11, r6, lsl #8
> >> + mov r11, r11, lsr #24 @ bits 23-16
> >> + mov r6, r6, lsr #24 @ bits 31-24
> >> cmn r0, #1
> >> #ifdef CONFIG_THUMB2_KERNEL
> >> moveq r0, #0x200000 @ set bit 21, mov to mvn instruction
> >> @@ -682,14 +689,20 @@ ARM_BE8(rev16 ip, ip)
> >> #ifdef CONFIG_CPU_ENDIAN_BE8
> >> @ in BE8, we load data in BE, but instructions still in LE
> >> bic ip, ip, #0xff000000
> >> - tst ip, #0x000f0000 @ check the rotation field
> >> + tst ip, #0x00040000 @ check the rotation field
> >> orrne ip, ip, r6, lsl #24 @ mask in offset bits 31-24
> >> + tst ip, #0x00080000 @ check the rotation field
> >> + orrne ip, ip, r11, lsl #24 @ mask in offset bits 23-16
> >> + tst ip, #0x000f0000 @ check the rotation field
> >> biceq ip, ip, #0x00004000 @ clear bit 22
> >> orreq ip, ip, r0 @ mask in offset bits 7-0
> >> #else
> >> bic ip, ip, #0x000000ff
> >> - tst ip, #0xf00 @ check the rotation field
> >> + tst ip, #0x400 @ check the rotation field
> >> orrne ip, ip, r6 @ mask in offset bits 31-24
> >> + tst ip, #0x800 @ check the rotation field
> >> + orrne ip, ip, r11 @ mask in offset bits 23-16
> >> + tst ip, #0xf00 @ check the rotation field
> >> biceq ip, ip, #0x400000 @ clear bit 22
> >> orreq ip, ip, r0 @ mask in offset bits 7-0
> >> #endif
> >> @@ -705,12 +718,12 @@ ENDPROC(__fixup_a_pv_table)
> >> 3: .long __pv_offset
> >>
> >> ENTRY(fixup_pv_table)
> >> - stmfd sp!, {r4 - r7, lr}
> >> + stmfd sp!, {r4 - r7, r11, lr}
> >> mov r3, #0 @ no offset
> >> mov r4, r0 @ r0 = table start
> >> add r5, r0, r1 @ r1 = table size
> >> bl __fixup_a_pv_table
> >> - ldmfd sp!, {r4 - r7, pc}
> >> + ldmfd sp!, {r4 - r7, r11, pc}
> >> ENDPROC(fixup_pv_table)
> >>
> >> .data
> >> --
> >> 1.8.3
> >>
> >>
> >>
> >> _______________________________________________
> >> linux-arm-kernel mailing list
> >> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> >> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
> >
> > .
> >
>

Next message: Liu Shixin: "[PATCH -next] dmaengine: sf-pdma: Remove set but not used variable "desc""
Previous message: Eric Dumazet: "Re: [PATCH] net: make netdev_wait_allrefs wake-able"
In reply to: Leizhen (ThunderTown): "Re: [PATCH v3 2/2] ARM: support PHYS_OFFSET minimum aligned at 64KiB boundary"
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