Re: [PATCH] x86/Xen: make use of IBPB controlling VM assist

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.02.2023 01:07, Boris Ostrovsky wrote:
> 
> On 2/14/23 6:53 PM, Boris Ostrovsky wrote:
>>
>> On 2/14/23 11:13 AM, Jan Beulich wrote:
>>
>>> --- a/arch/x86/kernel/cpu/bugs.c
>>> +++ b/arch/x86/kernel/cpu/bugs.c
>>> @@ -18,6 +18,8 @@
>>>   #include <linux/pgtable.h>
>>>   #include <linux/bpf.h>
>>>   +#include <xen/xen.h>
>>> +
>>>   #include <asm/spec-ctrl.h>
>>>   #include <asm/cmdline.h>
>>>   #include <asm/bugs.h>
>>> @@ -32,6 +34,7 @@
>>>   #include <asm/intel-family.h>
>>>   #include <asm/e820/api.h>
>>>   #include <asm/hypervisor.h>
>>> +#include <asm/xen/hypervisor.h>
>>>   #include <asm/tlbflush.h>
>>>     #include "cpu.h"
>>> @@ -934,7 +937,8 @@ do_cmd_auto:
>>>           break;
>>>         case RETBLEED_MITIGATION_IBPB:
>>> -        setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
>>> +        if (!xen_pv_domain() || xen_vm_assist_ibpb(true))
>>
>>
>> Is this going to compile without CONFIG_XEN?

Yes. The declaration of xen_vm_assist_ibpb() is visible (satisfying
the compiler) and DCE will eliminate the call to the function due to
xen_pv_domain() being constant "false" in that case, avoiding any
linking issues. The interesting case here really is building with
XEN but without XEN_PV: That's why I needed to put the function in
enlighten.c. This wouldn't be needed if xen_pv_domain() was also
constant "false" in that case (just like xen_pvh_domain() is when
!XEN_PVH).

>> I also think these two conditions should be wrapped into something to limit 
>> exposure of non-Xen code to Xen-specific primitives.

I would have done so, if I had any halfway sensible idea on how to
go about doing so in this particular case. In the absence of that it
looked okay-ish to me to reference Xen functions directly here.

> Oh, and this needs x86 maintainers.

Eventually yes. But I would prefer to sort the above question first
(which I'm sure would have been raised by them, in perhaps more
harsh a way), hence the initially limited exposure.

Jan