On Fri, May 16 2025, Daniel P. Berrangé <berra...@redhat.com> wrote:
> On Fri, May 16, 2025 at 04:51:34PM +0200, Cornelia Huck wrote: >> On Wed, May 14 2025, Daniel P. Berrangé <berra...@redhat.com> wrote: >> >> > 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. >> >> The kernel can tell us which ID register fields are writable (we won't >> generate properties if we don't.) For FEAT_xxx, this depends on how >> we'll end up handling them (maybe we should only expose them if all ID >> register bits backing them are actually writable.) >> >> What worries me a bit is that QEMU exposing a certain set of FEAT_xxx >> values could be interpreted as "those features are present, any other >> features aren't", while it is only the list of configurable features. >> >> Another issue: If libvirt is trying to baseline two cpus, it might end >> up creating a model that looks sane on paper, but migrations will fail >> because there are differences in non-writable bits. It would be much >> better if libvirt could detect beforehand that there was no common >> determinator. Not yet sure how to handle this. > > For "host" model that's probably not the end of the world. Apps have > already given up strong guarantee of migration compat by using 'host' > CPU and so in that context libvirt's feature comparison can assume > the underlying silicon is a match and just compare features. > > > In that sense the ability to list features and baseline two cpus > lets you guarantee that whatever CPU you boot the guest on, will > have at least those requested features. That's useful, even if it > does not give you a strong migration compat guarantee. > > Doing better would require info on non-writable features, and > possibly even that might not be sufficient to guarantee compat We'd probably want to use named models rather than 'host' for better generic handling, but that's a whole different can of worms that I'd prefer to keep closed right now. OTOH, 'host' with some features tweaked is already useful if you want to migrate across machines in a heterogeneous environment with known players (i.e. you know that the various machines only differ in features that you can actually configure.)
