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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 11 Jun 2024 10:59:56 -0700
Message-ID: <CAHk-=wipw+_LKyXpuq9X7suf1VDUX4wD6iCuxFJKm9g2+ntFkQ@mail.gmail.com>
Subject: Re: [PATCH 4/7] arm64: add 'runtime constant' support
To: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Anvin <hpa@zytor.com>, Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>, 
	Thomas Gleixner <tglx@linutronix.de>, Rasmus Villemoes <linux@rasmusvillemoes.dk>, 
	Josh Poimboeuf <jpoimboe@kernel.org>, Catalin Marinas <catalin.marinas@arm.com>, 
	Will Deacon <will@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, 
	"the arch/x86 maintainers" <x86@kernel.org>, linux-arm-kernel@lists.infradead.org, 
	linux-arch <linux-arch@vger.kernel.org>
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On Tue, 11 Jun 2024 at 10:48, Mark Rutland <mark.rutland@arm.com> wrote:
>
> Fair enough if that's a pain on x86, but we already have them on arm64, and
> hence using them is a smaller change there. We already have a couple of cases
> which uses MOVZ;MOVK;MOVK;MOVK sequence, e.g.
>
>         // in __invalidate_icache_max_range()
>         asm volatile(ALTERNATIVE_CB("movz %0, #0\n"
>                                     "movk %0, #0, lsl #16\n"
>                                     "movk %0, #0, lsl #32\n"
>                                     "movk %0, #0, lsl #48\n",
>                                     ARM64_ALWAYS_SYSTEM,
>                                     kvm_compute_final_ctr_el0)
>                      : "=r" (ctr));
>
> ... which is patched via the callback:
>
>         void kvm_compute_final_ctr_el0(struct alt_instr *alt,
>                                        __le32 *origptr, __le32 *updptr, int nr_inst)
>         {
>                 generate_mov_q(read_sanitised_ftr_reg(SYS_CTR_EL0),
>                                origptr, updptr, nr_inst);
>         }
>
> ... where the generate_mov_q() helper does the actual instruction generation.
>
> So if we only care about a few specific constants, we could give them their own
> callbacks, like kvm_compute_final_ctr_el0() above.

I'll probably only have another day until my mailbox starts getting
more pull requests (Mon-Tue outside the merge window is typically my
quiet time when I have time to go through old emails and have time for
private projects).

So I'll look at doing this for x86 and see how it works.

I do suspect that even then it's possibly more code with a
site-specific callback for each case, but maybe it would be worth it
just for the flexibility.

                   Linus