Re: [PATCHv2] x86/trampoline: Bypass compat mode in trampoline_start64() if not needed

[Sean Christopherson <seanjc@google.com>]

On Sun, Jan 07, 2024, Kirill A. Shutemov wrote:
> @@ -220,6 +222,33 @@ SYM_CODE_START(trampoline_start64)
>  	lidt	tr_idt(%rip)
>  	lgdt	tr_gdt64(%rip)
>  
> +	/* Check if paging mode has to be changed */
> +	movq	%cr4, %rax
> +	xorq	tr_cr4(%rip), %rax

This is buggy, tr_cr4 is only 4 bytes.  And even if tr_cr4 were 8 bytes, the reason
why nothing showed up in testing is also why only 4 bytes need to be XOR'd: the
upper 32 bits of the result are never consumed.

> +	andq	$X86_CR4_LA57, %rax

Nit, this can be TEST instead of AND, e.g. I was looking to see if RAX was used
anywhere in the flow.  And in theory it's possible a CPU could support uop fusing
for TEST+Jcc but not AND+Jcc, cause shaving a cycle in this code is obviously
super important ;-)

And as above, testl, not testq.

> +	jnz	.L_switch_paging
> +
> +	/* Paging mode is correct proceed in 64-bit mode */
> +
> +	LOCK_AND_LOAD_REALMODE_ESP lock_rip=1
> +
> +	movw	$__KERNEL_DS, %dx
> +	movl	%edx, %ss
> +	addl	$pa_real_mode_base, %esp
> +	movl	%edx, %ds
> +	movl	%edx, %es
> +	movl	%edx, %fs
> +	movl	%edx, %gs
> +
> +	movl	$pa_trampoline_pgd, %eax
> +	movq	%rax, %cr3
> +
> +	jmpq	*tr_start(%rip)
> +.L_switch_paging:
> +	/*
> +	 * To switch between 4- and 5-level paging modes, it is necessary
> +	 * to disable paging. This must be done in the compatibility mode.
> +	 */
>  	ljmpl	*tr_compat(%rip)
>  SYM_CODE_END(trampoline_start64)
>  
> -- 
> 2.41.0
>