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Re: [PATCH 4/4] x86/kexec: Disable FRED earlier in kexec_reloc()
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 24 Mar 2026 14:19:52 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 24 Mar 2026 13:20:02 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 19.03.2026 13:25, Andrew Cooper wrote:
> With FRED just as with IDT, it's unsafe to run the exception handlers after
> switching stack.
>
> To remove this unsafe window, %cr4 needs clearing earlier. In turn, we may
> need to switch to PCID 0 earlier too in order to be able to clear CR4.PCIDE.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> The pagetable switch cannot easily be moved to be earlier, as that leaves a
> period of time where FRED is enabled but all stack pointers point to
> non-existent mappings.
But the clearing of PCID could be moved earlier, I guess.
> --- a/xen/arch/x86/x86_64/kexec_reloc.S
> +++ b/xen/arch/x86/x86_64/kexec_reloc.S
> @@ -54,6 +54,21 @@ FUNC(kexec_reloc, PAGE_SIZE)
> lidt (%rsp)
> add $10, %rsp
>
> + /* Move to PCID 0 if necessary, as a prerequisite to clearing
> CR4.PCIDE */
> + mov %cr3, %rax
> + test $0xfff, %eax
> + jz 1f
This conditional looks not overly relevant on this path, and we'd get
more consistent overall behavior ...
> + and $~0xfff, %rax
> + mov %rax, %cr3
... if the implied TLB flush would happen in all cases. (Of course the
clearing of ...
> +1:
> +
> + /*
> + * Set CR4 to PAE only. This may disable FRED, which must happen
> + * before switching off Xen's stack.
> + */
> + mov $X86_CR4_PAE, %eax
> + mov %rax, %cr4
... PCIDE will flush again anyway.)
Jan
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