On Tue, Mar 19, 2019 at 02:20:01PM -0300, Eduardo Habkost wrote: > On Tue, Mar 19, 2019 at 04:16:14PM +0000, Daniel P. Berrangé wrote: > > On Tue, Mar 19, 2019 at 10:35:33PM +0800, Tao Xu wrote: > > > On 3/19/2019 8:16 PM, Daniel P. Berrangé wrote: > > > > The Cascadelake-Server CPU was added last year in > > > > > > > > commit c7a88b52f62b30c04158eeb07f73e3f72221b6a8 > > > > Author: Tao Xu <tao3...@intel.com> > > > > Date: Wed Sep 19 11:11:22 2018 +0800 > > > > > > > > i386: Add new model of Cascadelake-Server > > > > > > > > The commit message says that MSR based features are initially > > > > omitted from its definition: > > > > > > > > [quote] > > > > On Cascadelake, some capabilities (RDCL_NO, IBRS_ALL, RSBA, > > > > SKIP_L1DFL_VMENTRY and SSB_NO) are enumerated by MSR. > > > > These features rely on MSR based feature support patch. > > > > Will be added later after that patch's in. > > > > > > > > http://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg00074.html > > > > [/quote] > > > > [snip] > > > > > > Can anyone shed light on what the status is wrt exposing the MSR based > > > > features in Cascadelake-Server ? > > > > > > Hi, I sent the patch to add the MSR based features with the Update > > > stepping > > > patch together last year, and we discussed a lot about it: > > > > > > [1] https://patchwork.kernel.org/patch/10743427/ > > > > Ahh, I see, so none of the MSR based feature stuff is migratable. > > > > If migration isn't needed, then they can use "-cpu host" which gets all > > features automatically. > > > > If migration is needed, then the app is using a named CPU model, and no > > MSR based features should ever be used. > > > > Also means that libvirt must be careful to *not* enable MSR based > > features with "host-model" as this is supposde to be a migratable > > version of "host-passthrough" > > > > Is there any ETA on fixing things so that the MSR features can be > > used with named models ? > > > > Making it migratable and usable in named CPU models is the easy > part. This series is probably enough: > > http://lists.nongnu.org/archive/html/qemu-devel/2019-01/msg06701.html > ("[PATCH 0/2] i386: arch_capabilities fixes + migratability") > > I have just realized that I have left my own series fall through > the cracks. At least patch 1/2 is a bug fix, so I will merge it > for -rc1. > > The hard part is enabling the flag by default on existing CPU > models. We probably need to implement CPU model versioning in > QEMU 4.1 to do that properly. See discussion at: > > https://www.mail-archive.com/qemu-devel@nongnu.org/msg603291.html > ("Re: [Qemu-devel] [PATCH 2/2] i386: Add some MSR based features > on Cascadelake-Server CPU model")
Ok, so once the migration problem is fixed, IIUC the main blocker is that we want to be able to support use of Cascadelake-Server CPU model on older kernels that lacked the ARCH_CAPABILITIES support in KVM. We don't have a nice way to achieve that right now, but the versioned CPU models concept would allow for a solution. IOW, once migration is fixed, it is fine for mgmt apps to enable the use of the MSR features by turning them on manually for Cascadelake-Server if they know they don't care about older kernels. 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 :|
