Re: [PATCH v9 02/10] x86/bhi: Make clear_bhb_loop() effective on newer CPUs

[Jim Mattson]

On Fri, Apr 3, 2026 at 11:52 AM Pawan Gupta
<pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
>
> On Fri, Apr 03, 2026 at 11:10:08AM -0700, Jim Mattson wrote:
> > On Thu, Apr 2, 2026 at 5:32 PM Pawan Gupta
> > <pawan.kumar.gupta@xxxxxxxxxxxxxxx> wrote:
> > >
> > > As a mitigation for BHI, clear_bhb_loop() executes branches that overwrite
> > > the Branch History Buffer (BHB). On Alder Lake and newer parts this
> > > sequence is not sufficient because it doesn't clear enough entries. This
> > > was not an issue because these CPUs use the BHI_DIS_S hardware mitigation
> > > in the kernel.
> > >
> > > Now with VMSCAPE (BHI variant) it is also required to isolate branch
> > > history between guests and userspace. Since BHI_DIS_S only protects the
> > > kernel, the newer CPUs also use IBPB.
> > >
> > > A cheaper alternative to the current IBPB mitigation is clear_bhb_loop().
> > > But it currently does not clear enough BHB entries to be effective on newer
> > > CPUs with larger BHB. At boot, dynamically set the loop count of
> > > clear_bhb_loop() such that it is effective on newer CPUs too. Use the
> > > X86_FEATURE_BHI_CTRL feature flag to select the appropriate loop count.
> > >
> > > Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> > > ---
> > >  arch/x86/entry/entry_64.S            |  8 +++++---
> > >  arch/x86/include/asm/nospec-branch.h |  2 ++
> > >  arch/x86/kernel/cpu/bugs.c           | 13 +++++++++++++
> > >  3 files changed, 20 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > > index 3a180a36ca0e..bbd4b1c7ec04 100644
> > > --- a/arch/x86/entry/entry_64.S
> > > +++ b/arch/x86/entry/entry_64.S
> > > @@ -1536,7 +1536,9 @@ SYM_FUNC_START(clear_bhb_loop)
> > >         ANNOTATE_NOENDBR
> > >         push    %rbp
> > >         mov     %rsp, %rbp
> > > -       movl    $5, %ecx
> > > +
> > > +       movzbl    bhb_seq_outer_loop(%rip), %ecx
> > > +
> > >         ANNOTATE_INTRA_FUNCTION_CALL
> > >         call    1f
> > >         jmp     5f
> > > @@ -1556,8 +1558,8 @@ SYM_FUNC_START(clear_bhb_loop)
> > >          * This should be ideally be: .skip 32 - (.Lret2 - 2f), 0xcc
> > >          * but some Clang versions (e.g. 18) don't like this.
> > >          */
> > > -       .skip 32 - 18, 0xcc
> > > -2:     movl    $5, %eax
> > > +       .skip 32 - 20, 0xcc
> > > +2:     movzbl  bhb_seq_inner_loop(%rip), %eax
> > >  3:     jmp     4f
> > >         nop
> > >  4:     sub     $1, %eax
> > > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > > index 70b377fcbc1c..87b83ae7c97f 100644
> > > --- a/arch/x86/include/asm/nospec-branch.h
> > > +++ b/arch/x86/include/asm/nospec-branch.h
> > > @@ -548,6 +548,8 @@ DECLARE_PER_CPU(u64, x86_spec_ctrl_current);
> > >  extern void update_spec_ctrl_cond(u64 val);
> > >  extern u64 spec_ctrl_current(void);
> > >
> > > +extern u8 bhb_seq_inner_loop, bhb_seq_outer_loop;
> > > +
> > >  /*
> > >   * With retpoline, we must use IBRS to restrict branch prediction
> > >   * before calling into firmware.
> > > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > > index 83f51cab0b1e..2cb4a96247d8 100644
> > > --- a/arch/x86/kernel/cpu/bugs.c
> > > +++ b/arch/x86/kernel/cpu/bugs.c
> > > @@ -2047,6 +2047,10 @@ enum bhi_mitigations {
> > >  static enum bhi_mitigations bhi_mitigation __ro_after_init =
> > >         IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI) ? BHI_MITIGATION_AUTO : BHI_MITIGATION_OFF;
> > >
> > > +/* Default to short BHB sequence values */
> > > +u8 bhb_seq_outer_loop __ro_after_init = 5;
> > > +u8 bhb_seq_inner_loop __ro_after_init = 5;
> > > +
> > >  static int __init spectre_bhi_parse_cmdline(char *str)
> > >  {
> > >         if (!str)
> > > @@ -3242,6 +3246,15 @@ void __init cpu_select_mitigations(void)
> > >                 x86_spec_ctrl_base &= ~SPEC_CTRL_MITIGATIONS_MASK;
> > >         }
> > >
> > > +       /*
> > > +        * Switch to long BHB clear sequence on newer CPUs (with BHI_CTRL
> > > +        * support), see Intel's BHI guidance.
> > > +        */
> > > +       if (cpu_feature_enabled(X86_FEATURE_BHI_CTRL)) {
> > > +               bhb_seq_outer_loop = 12;
> > > +               bhb_seq_inner_loop = 7;
> > > +       }
> > > +
> >
> > How does this work for VMs in a heterogeneous migration pool that
> > spans the Alder Lake boundary? They can't advertise BHI_CTRL, because
> > it isn't available on all hosts in the migration pool, but they need
> > the long sequence when running on Alder Lake or newer.
>
> As we discussed elsewhere, support for migration pool is much more
> involved. It should be dealt in a separate QEMU/KVM focused series.
>
> A quickfix could be adding support for spectre_bhi=long that guests in a
> migration pool can use?

The simplest solution is to add "|
cpu_feature_enabled(X86_FEATURE_HYPERVISOR)" to the condition above.
If that is unacceptable for the performance of pre-Alder Lake
migration pools, you could define a CPUID or MSR bit that says
explicitly, "long BHB flush sequence needed," rather than trying to
intuit that property from the presence of BHI_CTRL. Like
IA32_ARCH_CAPABILITIES.SKIP_L1DFL_VMENTRY, the bit would only be set
by a hypervisor.

I am still skeptical of the need for MSR_VIRTUAL_ENUMERATION and
friends, unless there is a major guest OS out there that relies on
them.