On Wed, May 14, 2025 at 05:36:58PM +0200, Cornelia Huck wrote: > On Tue, May 13 2025, Daniel P. Berrangé <berra...@redhat.com> wrote: > > > On Mon, Apr 14, 2025 at 06:38:47PM +0200, Cornelia Huck wrote: > >> From: Eric Auger <eric.au...@redhat.com> > >> > >> If the interface for writable ID registers is available, expose uint64 > >> SYSREG properties for writable ID reg fields exposed by the host > >> kernel. Properties are named SYSREG_<REG>_<FIELD> with REG and FIELD > >> being those used in linux arch/arm64/tools/sysreg. This done by > >> matching the writable fields retrieved from the host kernel against the > >> generated description of sysregs. > >> > >> An example of invocation is: > >> -cpu host,SYSREG_ID_AA64ISAR0_EL1_DP=0x0 > >> which sets DP field of ID_AA64ISAR0_EL1 to 0. > > > > For the value you are illustrating 0x0 - is this implying that > > all the flags take an arbitrary integer hex value ? > > > > This would be different from x86, where CPU feature flags are > > a boolean on/off state. > > Most of the fields are 4 bits, the allowed values vary (there are also > some fields that are single bits, or wider.) The FEAT_xxx values (which > can be expressed via ID register fields, or a combination thereof) are > mostly boolean, but there are also some of them that can take values. > > We could cook up pseudo-features that are always on/off, but I don't > like that approach: they would be QEMU only, whereas the ID register > fields and FEAT_xxx features are all defined in the Arm documentation.
Fortunately from a libvirt POV we can likely expand our config to cope with hex values for arm features without too much trouble. > > An additional difference from x86 would be that FEAT_xxx featues are not > neccessarily configurable (only if the host kernel supports changing the > ID register field(s) backing the feature.) Is the kernel able to tell us which ones are configurable and which are not ? If so, it'd be helpful to expose this info in QAPI some place. > With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
