Re: [PATCH v10 1/3] arm64: Implement archrandom.h for ARMv8.5-RNG
From: Catalin Marinas <catalin.marinas@arm.com>
Date: 2020-01-16 11:02:59
On Wed, Jan 15, 2020 at 02:23:39PM -1000, Richard Henderson wrote:
On 1/15/20 4:26 AM, Catalin Marinas wrote:quoted
On Wed, Jan 15, 2020 at 11:07:20AM +0000, Mark Brown wrote:quoted
On Wed, Jan 15, 2020 at 10:24:21AM +0100, Ard Biesheuvel wrote:quoted
On Wed, 15 Jan 2020 at 10:16, Will Deacon [off-list ref] wrote:quoted
quoted
I see your argument, but I was just going on the side of consistency because we're continuing to expose other features as HWCAPs when the capability is just a proxy for the cpuid field. I was in favour of stopping the addition of such HWCAPs years ago, but I couldn't convince Catalin ;)quoted
quoted
The way I see it, we'll soon run out of HWCAP2 bits and then we'll have our hand forced.quoted
I don't have a strong opinion either way.Me either, or at least not enough to object to doing it - Will? Catalin?Until the ifunc resolver can work with CPUID, I think we should keep adding HWCAPn bits. We can revisit this with the toolchain people before introducing HWCAP3.Why would the ifunc resolver not be able to use HWCAP_CPUID?
It can indeed check the HWCAP_CPUID but I haven't seen any plans to implement the next part, actual use of an MRS instruction to read the corresponding ID_AA64* regs. This MRS emulation was requested by (some of) the toolchain people, even the architecture gained a feature to simplify the emulation, but followed by complete silence from the toolchain folk.
That said, speaking as a toolchain guy, you should conserve HWCAP2 bits so that, by preference, you do not need to introduce AT_HWCAP3. Or at least delay adding it.
We still have some time before AT_HWCAP3. Also, we have 32-bit spare in both HWCAP and HWCAP2 which we can use. IIRC we didn't go into the top 32-bit of HWCAP because we were still debating whether ILP32 makes sense (and now I'm 100% convinced it doesn't ;)). -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel