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Re: [Xen-devel] [PATCH v9 06/11] x86/entry: Organise the use of MSR_SPEC_CTRL at each entry/exit point

On 18/01/18 15:46, Andrew Cooper wrote:
> We need to be able to either set or clear IBRS in Xen context, as well as
> restore appropriate guest values in guest context.  See the documentation in
> asm-x86/spec_ctrl_asm.h for details.
> Writes to %cr3 are slower when SPEC_CTRL.IBRS is set, so the
> SPEC_CTRL_{ENTRY/EXIT}* positioning is important, and optimised for the
> expected common case of Xen not using IBRS itself.
> There is a semi-unrelated bugfix, where various asm_defn.h macros have a
> hidden dependency on PAGE_SIZE, which results in an assembler error if used in
> a .macro definition.
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

/sigh - It appears that I've missed two hunks from this patch, and as a
result, its subtly broken for HVM guests.

I'll post an incremental version when I've unbroken my automatic tests
and refreshed the XTF unit tests for these MSRs.

It is expected to be two extra mov's in the vmx/svm exit paths, so the
rest of the patch is ok for review in general.


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