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Re: [PATCH 03/22] x86/traps: Drop incorrect BUILD_BUG_ON() and comment in load_system_tables()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 12 Aug 2025 10:11:32 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 12 Aug 2025 08:11:42 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 08.08.2025 22:22, Andrew Cooper wrote:
> This was added erroneously by me.
>
> Hardware task switching does demand a TSS of at least 0x67 bytes, but that's
> not relevant in 64bit, and not relevant for Xen since commit
> 5d1181a5ea5e ("xen: Remove x86_32 build target.") in 2012.
>
> We already load a 0-length TSS in early_traps_init() demonstrating that it's
> possible.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> 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 f6ec5c9df522..cdc41248d4e9 100644
> --- a/xen/arch/x86/cpu/common.c
> +++ b/xen/arch/x86/cpu/common.c
> @@ -936,8 +936,6 @@ void load_system_tables(void)
> wrmsrl(MSR_ISST, (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) )
Well, the comment is wrong. Whether the BUILD_BUG_ON() itself is also wrong
depends on our intentions with the structure. Don't we need it to be that
size for everything (incl I/O bitmap) to work correctly elsewhere?
Jan
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