[PATCH] x86/traps: Drop incorrect BUILD_BUG_ON() and comment in load_system_tables()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

It is only the hardware task switching mechanism which cares about a TSS being
at least 0x67 bytes long.  Furthermore, since this check was added, the limit
is now 0x6b if CET-SS is active.  Nevertheless, task switches, being a relic
of the 32-bit days, aren't relevant to Xen.

LTR is is perfectly possible to load a shorter TSS, and indeed we will be
doing so with FRED active.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

In principle this is Fixes: 788844062561 ("x86/xpti: Don't leak TSS-adjacent
percpu data via Meltdown"), but I don't feel it's really a correct tag to
give.
---
 xen/arch/x86/cpu/common.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index e8d4ca3203be..698abb7086d8 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -900,8 +900,6 @@ void load_system_tables(void)
                wrmsrl(MSR_INTERRUPT_SSP_TABLE, (unsigned long)ist_ssp);
        }
 
-       BUILD_BUG_ON(sizeof(*tss) <= 0x67); /* Mandated by the architecture. */
-
        _set_tssldt_desc(gdt + TSS_ENTRY, (unsigned long)tss,
                         sizeof(*tss) - 1, SYS_DESC_tss_avail);
        if ( IS_ENABLED(CONFIG_PV32) )
-- 
2.39.5