[lkml]   [2018]   [Feb]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs
On 13/02/2018 09:15, David Woodhouse wrote:
>>> - if (!data)
>>> + if (!data && !spectre_v2_ibrs_all())
>>>   break;
>> This should check the value of IBRS_ALL in the VM, not in the host.
> No, it's host we want. If IBRS_ALL is set in the host, we set the
> actual hardware MSR once at boot time and never touch it again. The
> SPEC_CTRL MSR we expose to guests is purely a no-op fiction.
> If spectre_v2_ibrs_all() is true then KVM should *never* actually pass
> through or touch the real MSR.

That would be nice but unfortunately it's not possible. :(

The VM might actually not have IBRS_ALL, as usual the reason is
migration compatibility. In that case, that no-op fiction would be very
slow because the VM will actually do a lot of SPEC_CTRL writes.

So the right logic is:

- if the VM has IBRS_ALL, pass through the MSR when it is zero and
intercept writes when it is one (no writes should happen)

- if the VM doesn't have IBRS_ALL, do as we are doing now, independent
of what the host spectre_v2_ibrs_all() setting is.


 \ /
  Last update: 2018-02-13 10:59    [W:0.099 / U:0.428 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site