Re: [PATCH v2 06/15] x86/HVM: improve CET-IBT pruning of ENDBR

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.11.2023 23:28, Andrew Cooper wrote:
> On 24/11/2023 8:39 am, Jan Beulich wrote:
>> __init{const,data}_cf_clobber can have an effect only for pointers
>> actually populated in the respective tables. While not the case for SVM
>> right now, VMX installs a number of pointers only under certain
>> conditions. Hence the respective functions would have their ENDBR purged
>> only when those conditions are met. Invoke "pruning" functions after
>> having copied the respective tables, for them to install any "missing"
>> pointers.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> In theory Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, but see
> later.

Thanks. See below as well.

>> ---
>> This is largely cosmetic for present hardware, which when supporting
>> CET-IBT likely also supports all of the advanced VMX features for which
>> hook pointers are installed conditionally. The only case this would make
>> a difference there is when use of respective features was suppressed via
>> command line option (where available). For future hooks it may end up
>> relevant even by default, and it also would be if AMD started supporting
>> CET-IBT; right now it matters only for .pi_update_irte, as iommu_intpost
>> continues to default to off.
>>
>> Originally I had meant to put the SVM and VMX functions in presmp-
>> initcalls, but hvm/{svm,vmx}/built_in.o are linked into hvm/built_in.o
>> before hvm/hvm.o. And I don't think I want to fiddle with link order
>> here.
> 
> An alternative is the form I used for microcode, where start_{vmx,svm}()
> fills in fns, and doesn't have to fill in all hooks.
> 
> That will be more amenable to Kconfig-ing generally, and will probably
> be less fragile to getting forgotten.

You mean specifically "x86/ucode: Move vendor specifics back out of
early_microcode_init()", which looks to not have gone in yet? That's going
the opposite route (NULLing out hooks after copying), yet this looks to
still go against what you're asking for below. In how far this would reduce
the chance of being forgotten is not entirely clear to me right away, but I
think I see your point.

One (further) benefit of copying before editing would of course be that the
static struct instances then can be const.

>> --- a/xen/arch/x86/hvm/vmx/vmx.c
>> +++ b/xen/arch/x86/hvm/vmx/vmx.c
>> @@ -3033,6 +3033,30 @@ const struct hvm_function_table * __init
>>      return &vmx_function_table;
>>  }
>>  
>> +void __init prune_vmx(void)
>> +{
>> +    /*
>> +     * Now that vmx_function_table was copied, populate all function 
>> pointers
>> +     * which may have been left at NULL, for __initdata_cf_clobber to have 
>> as
>> +     * much of an effect as possible.
>> +     */
>> +    if ( !cpu_has_xen_ibt )
>> +        return;
>> +
>> +    vmx_function_table.set_descriptor_access_exiting =
>> +        vmx_set_descriptor_access_exiting;
>> +
>> +    vmx_function_table.update_eoi_exit_bitmap = vmx_update_eoi_exit_bitmap;
>> +    vmx_function_table.process_isr            = vmx_process_isr;
>> +    vmx_function_table.handle_eoi             = vmx_handle_eoi;
>> +
>> +    vmx_function_table.pi_update_irte = vmx_pi_update_irte;
>> +
>> +    vmx_function_table.deliver_posted_intr = vmx_deliver_posted_intr;
>> +    vmx_function_table.sync_pir_to_irr     = vmx_sync_pir_to_irr;
>> +    vmx_function_table.test_pir            = vmx_test_pir;
> 
> That said...
> 
> This (the hooks being conditional in the first place) is bogus to begin
> with.  Posted interrupts (or not) are a per-VM property even if we don't
> wire this up properly yet.  It will be forced to be done properly in
> order to support nested virt, as L0 Xen *must* comply with the settings
> chosen by the L1 hypervisor.
> 
> So the choice to use the hooks will have to come from per-vCPU state,
> and not from the conditional-ness of them.
> 
> Any chance I can talk you into instead making the hooks unconditional? 
> If not, someone (George was volunteering) is going to have to undo this
> in fairly short order.

As you can see from "x86/HVM: drop tsc_scaling.setup() hook" I actually
did consider removing the conditional parts, where sufficiently
straightforward. I'll take another close look for the other hooks, but
I'm not going to promise anything towards actually taking the alternative
route.

Jan