From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Xen-devel <xen-devel@lists.xenproject.org>,
"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH] x86/boot: Clean up the trampoline transition into Long mode
Date: Fri, 3 Jan 2020 14:36:53 +0100 [thread overview]
Message-ID: <d8abd43b-ac91-9eef-3c34-394ed9842342@suse.com> (raw)
In-Reply-To: <20200102145953.6503-1-andrew.cooper3@citrix.com>
On 02.01.2020 15:59, Andrew Cooper wrote:
> @@ -111,26 +109,6 @@ trampoline_protmode_entry:
> start64:
> /* Jump to high mappings. */
> movabs $__high_start, %rdi
> -
> -#ifdef CONFIG_INDIRECT_THUNK
> - /*
> - * If booting virtualised, or hot-onlining a CPU, sibling threads can
> - * attempt Branch Target Injection against this jmp.
> - *
> - * We've got no usable stack so can't use a RETPOLINE thunk, and are
> - * further than disp32 from the high mappings so couldn't use
> - * JUMP_THUNK even if it was a non-RETPOLINE thunk. Furthermore, an
> - * LFENCE isn't necessarily safe to use at this point.
> - *
> - * As this isn't a hotpath, use a fully serialising event to reduce
> - * the speculation window as much as possible. %ebx needs preserving
> - * for __high_start.
> - */
> - mov %ebx, %esi
> - cpuid
> - mov %esi, %ebx
> -#endif
> -
> jmpq *%rdi
I can see this being unneeded when running virtualized, as you said
in reply to Wei. However, for hot-onlining (when other CPUs may run
random vCPU-s) I don't see how this can safely be dropped. There's
no similar concern for S3 resume, as thaw_domains() happens only
after enable_nonboot_cpus().
Jan
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next prev parent reply other threads:[~2020-01-03 13:36 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-02 14:59 [Xen-devel] [PATCH] x86/boot: Clean up the trampoline transition into Long mode Andrew Cooper
2020-01-02 16:55 ` Wei Liu
2020-01-02 17:20 ` Andrew Cooper
2020-01-02 18:45 ` Wei Liu
2020-01-03 13:36 ` Jan Beulich [this message]
2020-01-03 13:44 ` Andrew Cooper
2020-01-03 13:52 ` Jan Beulich
2020-01-03 14:25 ` Andrew Cooper
2020-01-03 14:34 ` Jan Beulich
2020-01-03 18:55 ` Andrew Cooper
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