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Re: [PATCH 16/22] x86/boot: Adjust CR4 handling around ap_early_traps_init()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 14 Aug 2025 16:47:21 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 14 Aug 2025 14:47:32 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 08.08.2025 22:23, Andrew Cooper wrote:
> ap_early_traps_init() will shortly be setting CR4.FRED. This requires that
> cpu_info->cr4 is already set up, and that the enablement of CET doesn't
> truncate FRED back out because of it's 32bit logic.
>
> For __high_start(), defer re-loading XEN_MINIMAL_CR4 until after %rsp is set
> up and we can store the result in the cr4 field too.
>
> For s3_resume(), explicitly re-load XEN_MINIMAL_CR4. Later when loading all
> features, use the mmu_cr4_features variable which is how the rest of Xen
> performs this operation.
>
> No functional change, yet.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> --- a/xen/arch/x86/acpi/wakeup_prot.S
> +++ b/xen/arch/x86/acpi/wakeup_prot.S
> @@ -63,6 +63,14 @@ LABEL(s3_resume)
> pushq %rax
> lretq
> 1:
> +
> + GET_STACK_END(15)
> +
> + /* Enable minimal CR4 features. */
> + mov $XEN_MINIMAL_CR4, %eax
> + mov %rax, STACK_CPUINFO_FIELD(cr4)(%r15)
Strictly speaking this and ...
> --- a/xen/arch/x86/boot/x86_64.S
> +++ b/xen/arch/x86/boot/x86_64.S
> @@ -11,16 +11,19 @@ ENTRY(__high_start)
> mov %ecx,%gs
> mov %ecx,%ss
>
> - /* Enable minimal CR4 features. */
> - mov $XEN_MINIMAL_CR4,%rcx
> - mov %rcx,%cr4
> -
> mov stack_start(%rip),%rsp
>
> /* Reset EFLAGS (subsumes CLI and CLD). */
> pushq $0
> popf
>
> + GET_STACK_END(15)
> +
> + /* Enable minimal CR4 features. */
> + mov $XEN_MINIMAL_CR4, %eax
> + mov %rax, STACK_CPUINFO_FIELD(cr4)(%r15)
... this could be 32-bit stores, even in the longer run.
Jan
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