Re: [PATCH v2 2/2] x86/spec: adjust logic to logic that elides lfence

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 18/04/2024 4:52 pm, Roger Pau Monne wrote:
> It's currently too restrictive by just checking whether there's a BHB clearing
> sequence selected.  It should instead check whether BHB clearing is used on
> entry from PV or HVM specifically.
>
> Switch to use opt_bhb_entry_{pv,hvm} instead, and then remove cpu_has_bhb_seq
> since it no longer has any users.
>
> Reported-by: Jan Beulich <jbeulich@suse.com>
> Fixes: 954c983abcee ('x86/spec-ctrl: Software BHB-clearing sequences')
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
> Changes since v1:
>  - New in this version.
>
> There (possibly) still a bit of overhead for dom0 if BHB clearing is not used
> for dom0, as Xen would still add the lfence if domUs require it.

This is the note about dom0 that I made on the previous patch.

"protect dom0" only has any effect if the appropriate foo_pv or foo_hvm
is also selected.  It's not possible to express "protect dom0 but not
domU of $TYPE".

This early on boot we have no idea whether dom0 is going to be PV or
HVM.  We could in principle figure it out by peeking at dom0's ELF
notes, but that needs a lot of rearranging of __start_xen() to do safely.

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>