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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.03.2025 19:33, Andrew Cooper wrote:
> It is only the hardware task switching mechanism which cares about a TSS being
> at least 0x67 bytes long.

I/O bitmap accesses are where this particular limit comes into play. For
32-bit task switching a slightly shorter one would still do, I think?

>  Furthermore, since this check was added, the limit is now 0x6b if CET-SS is
> active.

Which isn't reflected at all in struct tss64: Aiui that's an addition to the
32-bit TSS only.

>  Nevertheless, task switches, being a relic
> of the 32-bit days, aren't relevant to Xen.

Yet as per above I/O bitmap handling is, even if all we put there is
IOBMP_INVALID_OFFSET.

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

Well, yes; it is my understanding that shorter ones can already be loaded
without FRED.

> --- 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);

All of the above said, the removal worries me primarily with the sizeof()
still in use here.

Jan