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Re: [PATCH 3/3] x86/entry: Use PUSH_AND_CLEAR_GPRS and drop SAVE_ALL
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 7 Jul 2026 09:16:16 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 07 Jul 2026 07:16:34 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 06.07.2026 17:34, Andrew Cooper wrote:
> PUSH_AND_CLEAR_GPRS is shorter than SAVE_ALL in terms of emitted code.
Could patch 1 perhaps also gain this sentence?
> lstar_enter() and cstar_enter() do not need CLD. The SYSCALL instruction
> sanitises flags based on MSR_SYSCALL_MASK. For all other cases, place the CLD
> instruction next to STAC/CLAC so the flag handling is together.
And really the earlier the better. Relevant perhaps for early_page_fault(),
which has no CLAC.
> Get rid of the compat=1 special case for PV32. It's not obviously a win, and
> PV32 is getting increasingly rare these days.
>
> No functional change.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> Bloat-o-meter reports:
> add/remove: 0/0 grow/shrink: 0/12 up/down: 0/-518 (-518)
> Function old new delta
> symbols_names 135277 135276 -1
> symbols_offsets 53664 53660 -4
Interesting. Likely an artifact of padding going away between two functions.
Which likely would change again if the SLS patches would finally land
("x86: guard against straight-line speculation past JMP or RET in assembly
files" here in particular).
Btw, as it occurs to me while mentioning SLS: Judging from the patches I
have, IRET looks to be immune, albeit I can't find any statement in the doc.
What about ERET{S,U}, uses of which we've gained only relatively recently?
Jan
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