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Re: [PATCH 2/2] x86/pv: Provide better SYSCALL backwards compatibility in FRED mode
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 31 Mar 2026 08:31:18 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 31 Mar 2026 07:31:40 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 27/03/2026 8:50 am, Jan Beulich wrote:
> On 26.03.2026 22:05, Andrew Cooper wrote:
>> On 26/03/2026 9:14 am, Jan Beulich wrote:
>>> Don't you also need to set TRAP_syscall here, for the new code in
>>> eretu_exit_to_guest to actually make a difference?
>> It is create_bounce_frame() which sets up TRAP_syscall.
> Hmm, right. I was misled by {l,c}star_enter and sysenter_entry setting
> the flag explicitly. That looks to be necessary only for the pv_hypercall()
> path out of lstar_enter; everything else goes through create_bounce_frame().
Oh, that's a can of worms in and of itself.
The hypercall page preserves %rcx/%r11 around the SYSCALL instruction,
but that's not captured in
https://xenbits.xen.org/docs/latest/guest-guide/x86/hypercall-abi.html
Given that we're now explicitly supporting the use of hypercalls without
the hypercall page for CFI hardening reasons, I guess I need to fix that.
~Andrew
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