Re: [RFC PATCH v1 55/57] arm64: TRAMP_VALIAS is no longer compile-time constant

Ryan Roberts
2024-10-14T13:22:48Z

On 14/10/2024 12:21, Ard Biesheuvel wrote:
> Hi Ryan,
>
> On Mon, 14 Oct 2024 at 13:02, Ryan Roberts <ryan.roberts@arm.com> wrote:
>>
>> When boot-time page size is in operation, TRAMP_VALIAS is no longer a
>> compile-time constant, because the VA of a fixmap slot depends upon
>> PAGE_SIZE.
>>
>> Let's handle this by instead exporting the slot index,
>> FIX_ENTRY_TRAMP_BEGIN,to assembly, then do the TRAMP_VALIAS calculation
>> per page size and use alternatives to decide which variant to activate.
>>
>> Note that for the tramp_map_kernel case, we are one instruction short of
>> space in the vector to have NOPs for all 3 page size variants. So we do
>> if/else for 16K/64K and branch around it for the 4K case. This saves 2
>> instructions.
>>
>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>> ---
>>
>> ***NOTE***
>> Any confused maintainers may want to read the cover note here for context:
>> https://lore.kernel.org/all/20241014105514.3206191-1-ryan.roberts@arm.com/
>>
>> arch/arm64/kernel/asm-offsets.c | 2 +-
>> arch/arm64/kernel/entry.S | 50 ++++++++++++++++++++++++++-------
>> 2 files changed, 41 insertions(+), 11 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
>> index f32b8d7f00b2a..c45fa3e281884 100644
>> --- a/arch/arm64/kernel/asm-offsets.c
>> +++ b/arch/arm64/kernel/asm-offsets.c
>> @@ -172,7 +172,7 @@ int main(void)
>> DEFINE(ARM64_FTR_SYSVAL, offsetof(struct arm64_ftr_reg, sys_val));
>> BLANK();
>> #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
>> - DEFINE(TRAMP_VALIAS, TRAMP_VALIAS);
>> + DEFINE(FIX_ENTRY_TRAMP_BEGIN, FIX_ENTRY_TRAMP_BEGIN);
>> #endif
>> #ifdef CONFIG_ARM_SDE_INTERFACE
>> DEFINE(SDEI_EVENT_INTREGS, offsetof(struct sdei_registered_event, interrupted_regs));
>> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
>> index 7ef0e127b149f..ba47dc8672c04 100644
>> --- a/arch/arm64/kernel/entry.S
>> +++ b/arch/arm64/kernel/entry.S
>> @@ -101,11 +101,27 @@
>> .org .Lventry_start\@ + 128 // Did we overflow the ventry slot?
>> .endm
>>
>> +#define TRAMP_VALIAS(page_shift) (FIXADDR_TOP - (FIX_ENTRY_TRAMP_BEGIN << (page_shift)))
>> +
>> .macro tramp_alias, dst, sym
>> - .set .Lalias\@, TRAMP_VALIAS + \sym - .entry.tramp.text
>> - movz \dst, :abs_g2_s:.Lalias\@
>> - movk \dst, :abs_g1_nc:.Lalias\@
>> - movk \dst, :abs_g0_nc:.Lalias\@
>> +alternative_if ARM64_USE_PAGE_SIZE_4K
>> + .set .Lalias4k\@, TRAMP_VALIAS(ARM64_PAGE_SHIFT_4K) + \sym - .entry.tramp.text
>> + movz \dst, :abs_g2_s:.Lalias4k\@
>> + movk \dst, :abs_g1_nc:.Lalias4k\@
>> + movk \dst, :abs_g0_nc:.Lalias4k\@
>> +alternative_else_nop_endif
>> +alternative_if ARM64_USE_PAGE_SIZE_16K
>> + .set .Lalias16k\@, TRAMP_VALIAS(ARM64_PAGE_SHIFT_16K) + \sym - .entry.tramp.text
>> + movz \dst, :abs_g2_s:.Lalias16k\@
>> + movk \dst, :abs_g1_nc:.Lalias16k\@
>> + movk \dst, :abs_g0_nc:.Lalias16k\@
>> +alternative_else_nop_endif
>> +alternative_if ARM64_USE_PAGE_SIZE_64K
>> + .set .Lalias64k\@, TRAMP_VALIAS(ARM64_PAGE_SHIFT_64K) + \sym - .entry.tramp.text
>> + movz \dst, :abs_g2_s:.Lalias64k\@
>> + movk \dst, :abs_g1_nc:.Lalias64k\@
>> + movk \dst, :abs_g0_nc:.Lalias64k\@
>> +alternative_else_nop_endif
>
> Since you're changing these, might as well drop the middle movk as the
> fixmap is now always in the top 2 GiB of the VA space.
>
> However, wouldn't it be better to reuse the existing callback
> alternative stuff that Marc added for KVM?

Yes, I agree. Mark suggested the same thing when we were talking the other day
too. I'll definitely use the callbacks for next version, but I didn't want to
hold up the RFC any further - I'd already spent way too much time polishing.

>
> Same applies below, I reckon.
>
>> .endm
>>
>> /*
>> @@ -627,16 +643,30 @@ SYM_CODE_END(ret_to_user)
>> bic \tmp, \tmp, #USER_ASID_FLAG
>> msr ttbr1_el1, \tmp
>> #ifdef CONFIG_QCOM_FALKOR_ERRATUM_1003
>> -alternative_if ARM64_WORKAROUND_QCOM_FALKOR_E1003
>> +alternative_if_not ARM64_WORKAROUND_QCOM_FALKOR_E1003
>> + b .Lskip_falkor_e1003\@
>> +alternative_else_nop_endif
>> /* ASID already in \tmp[63:48] */
>> - movk \tmp, #:abs_g2_nc:(TRAMP_VALIAS >> 12)
>> - movk \tmp, #:abs_g1_nc:(TRAMP_VALIAS >> 12)
>> - /* 2MB boundary containing the vectors, so we nobble the walk cache */
>> - movk \tmp, #:abs_g0_nc:((TRAMP_VALIAS & ~(SZ_2M - 1)) >> 12)
>> +alternative_if ARM64_USE_PAGE_SIZE_4K
>> + movk \tmp, #:abs_g2_nc:(TRAMP_VALIAS(ARM64_PAGE_SHIFT_4K) >> 12)
>> + movk \tmp, #:abs_g1_nc:(TRAMP_VALIAS(ARM64_PAGE_SHIFT_4K) >> 12)
>> + movk \tmp, #:abs_g0_nc:((TRAMP_VALIAS(ARM64_PAGE_SHIFT_4K) & ~(SZ_2M - 1)) >> 12)
>> + b .Lfinish_falkor_e1003\@
>> +alternative_else_nop_endif
>> +alternative_if ARM64_USE_PAGE_SIZE_16K
>> + movk \tmp, #:abs_g2_nc:(TRAMP_VALIAS(ARM64_PAGE_SHIFT_16K) >> 12)
>> + movk \tmp, #:abs_g1_nc:(TRAMP_VALIAS(ARM64_PAGE_SHIFT_16K) >> 12)
>> + movk \tmp, #:abs_g0_nc:((TRAMP_VALIAS(ARM64_PAGE_SHIFT_16K) & ~(SZ_2M - 1)) >> 12)
>> +alternative_else /* ARM64_USE_PAGE_SIZE_64K */
>> + movk \tmp, #:abs_g2_nc:(TRAMP_VALIAS(ARM64_PAGE_SHIFT_64K) >> 12)
>> + movk \tmp, #:abs_g1_nc:(TRAMP_VALIAS(ARM64_PAGE_SHIFT_64K) >> 12)
>> + movk \tmp, #:abs_g0_nc:((TRAMP_VALIAS(ARM64_PAGE_SHIFT_64K) & ~(SZ_2M - 1)) >> 12)
>> +alternative_endif
>> +.Lfinish_falkor_e1003\@:
>> isb
>> tlbi vae1, \tmp
>> dsb nsh
>> -alternative_else_nop_endif
>> +.Lskip_falkor_e1003\@:
>> #endif /* CONFIG_QCOM_FALKOR_ERRATUM_1003 */
>> .endm
>>
>> --
>> 2.43.0
>>