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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 16:56:54 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 14 Aug 2025 14:57:01 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 14.08.2025 16:54, Andrew Cooper wrote:
> On 14/08/2025 3:47 pm, Jan Beulich wrote:
>> On 08.08.2025 22:23, Andrew Cooper wrote:
>>> --- 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.
>
> ... no, they can't.
>
> The store also serves to clear out stale X86_CR4_FRED, prior to FRED
> getting reconfigured again.
>
> fatal_trap() uses info->cr4 to decide whether it's safe to look at the
> extended FRED metadata. Strictly speaking I probably ought to read the
> real CR4 (in read_registers too), but using a 32bit store here would
> extend a 1-instruction window into quite a larger window where exception
> handling would not work quite right.
Oh, I see. Mind me asking to add brief comments there to this effect?
Jan
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