Re: [PATCH 3/3] x86/pv: Compile out compat_gdt in !CONFIG_PV builds

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/04/2020 16:47, Jan Beulich wrote:
> On 20.04.2020 16:39, Andrew Cooper wrote:
>> On 20/04/2020 15:12, Jan Beulich wrote:
>>> On 17.04.2020 17:50, Andrew Cooper wrote:
>>>> There is no need for the Compat GDT if there are no 32bit PV guests.  This
>>>> saves 4k per online CPU
>>>>
>>>> Bloat-o-meter reports the following savings in Xen itself:
>>>>
>>>>   add/remove: 0/3 grow/shrink: 1/4 up/down: 7/-4612 (-4605)
>>>>   Function                                     old     new   delta
>>>>   cpu_smpboot_free                            1249    1256      +7
>>>>   per_cpu__compat_gdt_l1e                        8       -      -8
>>>>   per_cpu__compat_gdt                            8       -      -8
>>>>   init_idt_traps                               442     420     -22
>>>>   load_system_tables                           414     364     -50
>>>>   trap_init                                    444     280    -164
>>>>   cpu_smpboot_callback                        1255     991    -264
>>>>   boot_compat_gdt                             4096       -   -4096
>>>>   Total: Before=3062726, After=3058121, chg -0.15%
>>>>
>>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>>> ---
>>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>>> CC: Wei Liu <wl@xxxxxxx>
>>>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>>>
>>>> The increase in cpu_smpboot_free() appears to be a consequence of a totally
>>>> different layout of basic blocks.
>>>> ---
>>>>  xen/arch/x86/cpu/common.c |  5 +++--
>>>>  xen/arch/x86/desc.c       |  2 ++
>>>>  xen/arch/x86/smpboot.c    |  5 ++++-
>>>>  xen/arch/x86/traps.c      | 10 +++++++---
>>>>  4 files changed, 16 insertions(+), 6 deletions(-)
>>>>
>>>> diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
>>>> index 1b33f1ed71..7b093cb421 100644
>>>> --- a/xen/arch/x86/cpu/common.c
>>>> +++ b/xen/arch/x86/cpu/common.c
>>>> @@ -752,8 +752,9 @@ void load_system_tables(void)
>>>>  
>>>>    _set_tssldt_desc(gdt + TSS_ENTRY, (unsigned long)tss,
>>>>                     sizeof(*tss) - 1, SYS_DESC_tss_avail);
>>>> -  _set_tssldt_desc(compat_gdt + TSS_ENTRY, (unsigned long)tss,
>>>> -                   sizeof(*tss) - 1, SYS_DESC_tss_busy);
>>>> +  if ( IS_ENABLED(CONFIG_PV32) )
>>>> +          _set_tssldt_desc(compat_gdt + TSS_ENTRY, (unsigned long)tss,
>>>> +                           sizeof(*tss) - 1, SYS_DESC_tss_busy);
>>> Wouldn't this better be "if ( opt_pv32 )"? Also elsewhere then.
>> Doing it like this specifically ensures that there is never a case where
>> things are half configured.
> But this way you set up something in the GDT that's never going
> to be used when "pv=no-32". Why leave a TSS accessible that we
> don't need?

Defence in depth.

Having it only partially set up is more likely to fail in a security
relevant way, than having it fully set up.

This particular example is poor.  There is no need to have the TSS in
either GDT after the `ltr` instruction at boot for 64bit, because we
don't task switch, but ISTR you requesting that this stayed as-was for
consistency.  (For 32bit Xen, it was strictly necessary for the #DF task
switch to work.)

However, the other logic, particularly the cached l1e pointing at the
percpu compat_gdt is more liable to go wrong in interesting ways.

~Andrew