Re: [PATCH 2/2] x86/pv: Provide better SYSCALL backwards compatibility in FRED mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.03.2026 18:02, Andrew Cooper wrote:
> In FRED mode, the SYSCALL instruction does not modify %rcx/%r11.  Software
> using SYSCALL spills %rcx/%r11 around the invocation, which is why FRED not
> doing this goes largely unnoticed.
> 
> However, consider the following migration scenario:
> 
>  * VM suspends.  Hypercall, so SYSCALL, %rcx/%r11 left unmodified
>  * VM moves to a non-FRED system
>  * Xen resumes the VM with a real SYSRET instruction
> 
> Instead of resuming at the instruction following the SYSCALL instruction, the
> VM is resumed at whatever dead value was in %rcx.

Would it? In restore_all_guest we load %r11 and %rcx from the stack
frame's EFLAGS and RIP fields. If we didn't, various other things wouldn't
work either.

> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -2405,6 +2405,8 @@ void asmlinkage entry_from_pv(struct cpu_user_regs 
> *regs)
>  
>              regs->ssx = l ? FLAT_KERNEL_SS   : FLAT_USER_SS32;
>              regs->csx = l ? FLAT_KERNEL_CS64 : FLAT_USER_CS32;
> +            regs->rcx = regs->rip;
> +            regs->r11 = regs->rflags;

Don't you also need to set TRAP_syscall here, for the new code in
eretu_exit_to_guest to actually make a difference? (There actually is
a paragraph about this in the comment out of context above, which then
may also want adjusting.)

Further a question as to limiting overhead: Doing this on every SYSCALL
entry ...

> @@ -26,7 +27,16 @@ FUNC(entry_FRED_R3, 4096)
>  END(entry_FRED_R3)
>  
>  FUNC(eretu_exit_to_guest)
> -        POP_GPRS
> +        /*
> +         * PV guests aren't aware of FRED.  If Xen in IDT mode would have 
> used
> +         * a SYSRET instruction, preserve the legacy behaviour for %rcx/%r11
> +         */
> +        testb   $TRAP_syscall >> 8, UREGS_entry_vector + 1(%rsp)
> +
> +        POP_GPRS /* Preserves flags */
> +
> +        cmovnz  EFRAME_rip(%rsp), %rcx
> +        cmovnz  EFRAME_eflags(%rsp), %r11

... and every exit-to-guest isn't very nice when concern is about just the
specific case of migrating FRED -> non-FRED. Couldn't we instead make the
adjustment when generating the save record for the register state of the
vCPU?

Jan