Hi Kyrill, > -----Original Message----- > From: Kyrylo Tkachov <ktkac...@nvidia.com> > Sent: Thursday, June 27, 2024 9:58 AM > To: gcc-patches@gcc.gnu.org > Cc: Richard Earnshaw <richard.earns...@arm.com>; Richard Sandiford > <richard.sandif...@arm.com> > Subject: [PATCH] aarch64: Remove RNG and MTE from -mcpu=neoverse-v2 > > Hi all, > > According to the TRM for Neoverse V2 the Memory Tagging and RNG features > are optional configurations of the core and may not always be present. > Therefore -mcpu=neoverse-v2 shouldn't enable them, similar to how the crypto > extensions aren’t enabled by default.
RNG is indeed optional, however Memory Tagging is not. The table is a bit cryptic but it distinguishes between three states: "supported", "supported using configurable option" and "unsupported". RNG is supported with configurable option, but MTE is "supported" (this wording seems to be used for things that are mandatory.). If you look in table 2-7 of the TRM it states "The Neoverse-V2 core always implements MTE". This can be confirmed by looking at the table A-204: ID_AA64PFR1_EL1 bit descriptions. For MTE the register cannot be 0b000, i.e. even though MTE is architecturally optional, it's not valid for a Neoverse-V2 core not to have MTE. This means that the compiler should declare support for MTE as at the CPU level it's always supported. Kind Regards, Tamar > > Bootstrapped and tested on aarch64-none-linux-gnu. > Does this reasoning make sense? > Thanks, > Kyrill