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    SubjectRe: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure
    On Wed, Jan 31, 2018 at 02:04:49PM +0000, Dr. David Alan Gilbert wrote:
    > * Borislav Petkov (bp@suse.de) wrote:
    > > On Wed, Jan 31, 2018 at 12:30:36PM +0000, Dr. David Alan Gilbert wrote:
    > > > Indeed, it's only for this weird case where you suddenly need to change
    > > > it.
    > >
    > > No, there's more:
    > >
    > > .name = "Broadwell-noTSX",
    > > .name = "Haswell-noTSX",
    >
    > Haswell came out and we made the CPU definition, and then got a
    > microcode update that removed the feature.
    >
    > So the common feature of noTSX and IBRS is that they're the only two
    > cases where a CPU has released and then the flags have changed later.

    Also, if anybody don't like it, users can already specify, e.g.,
    "Broadwell,-hle,-rtm" or "Skylake,+spec_ctrl".

    QEMU only adds have the -noTSX and -IBRS CPU for convenience of
    management systems that don't know how to check/configure
    individual CPU features. We're working with libvirt and
    OpenStack folks to make this kind of trick unnecessary.

    --
    Eduardo

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