Messages in this thread Patch in this message | | | Subject | Re: [PATCH v2] arm64: kvm: Use has_vhe() instead of hyp_alternate_select() | From | Shanker Donthineni <> | Date | Mon, 6 Mar 2017 07:45:38 -0600 |
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Hi Marc,
On 03/06/2017 02:34 AM, Marc Zyngier wrote: > Hi Shanker, > > On Mon, Mar 06 2017 at 2:33:18 am GMT, Shanker Donthineni <shankerd@codeaurora.org> wrote: >> Now all the cpu_hwcaps features have their own static keys. We don't >> need a separate function hyp_alternate_select() to patch the vhe/nvhe >> code. We can achieve the same functionality by using has_vhe(). It >> improves the code readability, uses the jump label instructions, and >> also compiler generates the better code with a fewer instructions. > How do you define "better"? Which compiler? Do you have any benchmarking data? I'm using gcc version 5.2.0. With has_vhe() it shows the smaller code size as shown below. I tried to benchmark the code changes using Cristiffer's microbench tool, but not seeing a noticeable difference on QDF2400 platform.
hyp_alternate_select() uses BR/BLR instructions to patch vhe/mvhe code, which is not good for branch prediction purpose. compiler treats patched code as a function call, so the contents of the registers x0-x18 are not reusable after vhe/nvhe call.
Current code: arch/arm64/kvm/hyp/switch.o: file format elf64-littleaarch64
Sections: Idx Name Size VMA LMA File off Algn 0 .text 00000000 0000000000000000 0000000000000000 00000040 2**0 CONTENTS, ALLOC, LOAD, READONLY, CODE 1 .data 00000000 0000000000000000 0000000000000000 00000040 2**0 CONTENTS, ALLOC, LOAD, DATA 2 .bss 00000000 0000000000000000 0000000000000000 00000040 2**0 ALLOC 3 .hyp.text 00000550 0000000000000000 0000000000000000 00000040 2**3 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
New code: arch/arm64/kvm/hyp/switch.o: file format elf64-littleaarch64
Sections: Idx Name Size VMA LMA File off Algn 0 .text 00000000 0000000000000000 0000000000000000 00000040 2**0 CONTENTS, ALLOC, LOAD, READONLY, CODE 1 .data 00000000 0000000000000000 0000000000000000 00000040 2**0 CONTENTS, ALLOC, LOAD, DATA 2 .bss 00000000 0000000000000000 0000000000000000 00000040 2**0 ALLOC 3 .hyp.text 00000488 0000000000000000 0000000000000000 00000040 2**3 CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
>> Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org> >> --- >> v2: removed 'Change-Id: Ia8084189833f2081ff13c392deb5070c46a64038' from commit >> >> arch/arm64/kvm/hyp/debug-sr.c | 12 ++++++---- >> arch/arm64/kvm/hyp/switch.c | 50 +++++++++++++++++++----------------------- >> arch/arm64/kvm/hyp/sysreg-sr.c | 23 +++++++++---------- >> 3 files changed, 43 insertions(+), 42 deletions(-) >> >> diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c >> index f5154ed..e5642c2 100644 >> --- a/arch/arm64/kvm/hyp/debug-sr.c >> +++ b/arch/arm64/kvm/hyp/debug-sr.c >> @@ -109,9 +109,13 @@ static void __hyp_text __debug_save_spe_nvhe(u64 *pmscr_el1) >> dsb(nsh); >> } >> >> -static hyp_alternate_select(__debug_save_spe, >> - __debug_save_spe_nvhe, __debug_save_spe_vhe, >> - ARM64_HAS_VIRT_HOST_EXTN); >> +static void __hyp_text __debug_save_spe(u64 *pmscr_el1) >> +{ >> + if (has_vhe()) >> + __debug_save_spe_vhe(pmscr_el1); >> + else >> + __debug_save_spe_nvhe(pmscr_el1); >> +} > I have two worries about this kind of thing: > - Not all compilers do support jump labels, leading to a memory access > on each static key (GCC 4.8, for example). This would immediately > introduce a pretty big regression > - The hyp_alternate_select() method doesn't introduce a fast/slow path > duality. Each path has the exact same cost. I'm not keen on choosing > what is supposed to be the fast path, really. Yes, it'll require a runtime check if the compiler doesn't support ASM GOTO labels. Agree, hyp_alternate_select() has a constant branch over head but it might cause a branch prediction penality.
> Thanks, > > M.
-- Shanker Donthineni Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
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