Re: [PATCH v3 13/22] x86/traps: Enable FRED when requested

[Jan Beulich <jbeulich@suse.com>]

On 04.10.2025 00:53, Andrew Cooper wrote:
> With the shadow stack and exception handling adjustements in place, we can now
> activate FRED when appropriate.  Note that opt_fred is still disabled by
> default.
> 
> Introduce init_fred() to set up all the MSRs relevant for FRED.  FRED uses
> MSR_STAR (entries from Ring3 only), and MSR_FRED_SSP_SL0 aliases MSR_PL0_SSP
> when CET-SS is active.  Otherwise, they're all new MSRs.
> 
> With init_fred() existing, load_system_tables() and legacy_syscall_init()
> should only be used when setting up IDT delivery.  Insert ASSERT()s to this
> effect, and adjust the various *_init() functions to make this property true.
> 
> Per the documentation, percpu_early_traps_init() is responsible for switching
> off the boot GDT, which needs doing even in FRED mode.
> 
> Finally, set CR4.FRED in traps_init()/percpu_early_traps_init().
> 
> Xen can now boot in FRED mode up until starting a PV guest, where it faults
> because IRET is not permitted to change privilege.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>