Re: [PATCH v9 08/10] x86/mm/pti: Introduce a kernel/user CR3 software signal

[Juri Lelli <juri.lelli@redhat.com>]

Hello!

On 05/05/26 10:23, Valentin Schneider wrote:
> Later commits will rely on being able to check whether a remote CPU is
> using the kernel or the user CR3.
> 
> This software signal needs to be updated before the actual CR3 write, IOW
> it always immediately precedes it:
> 
>   KERNEL_CR3_LOADED := 1
>   SWITCH_TO_KERNEL_CR3
>   [...]
>   KERNEL_CR3_LOADED := 0
>   SWITCH_TO_USER_CR3
> 
> The variable also gets mapped into the user space visible pages.
> I tried really hard not to do that, and at some point had something mostly
> working with having an alias to it through the cpu_entry_area accessed like
> so before the switch to the kernel CR3:
> 
> 	subq $10, %rsp
> 	sgdt (%rsp)
> 	movq 2(%rsp), \scratch_reg /* GDT address */
> 	addq $10, %rsp
> 
> 	movl $1, CPU_ENTRY_AREA_kernel_cr3(\scratch_reg)
> 
> however this explodes when running 64-bit user code that invokes SYSCALL,
> since the scratch reg is %rsp itself, and I figured this was enough headaches.
> 
> This will only be really useful for NOHZ_FULL CPUs, but it should be
> cheaper to unconditionally update a never-used per-CPU variable living in
> its own cacheline than to check a shared cpumask such as
>   housekeeping_cpumask(HK_TYPE_KERNEL_NOISE)
> at every entry.
> 
> Signed-off-by: Valentin Schneider <vschneid@redhat.com>
> ---

Just fyi, I had to do the below to make clang happy with this.

Best,
Juri
---
diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index deb8224b5ee48..07fabf762f700 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -186,7 +186,7 @@ For 32-bit we have the following conventions - kernel is built with
 
 .macro SWITCH_TO_KERNEL_CR3 scratch_reg:req
 	ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
-	NOTE_CR3_SWITCH \scratch_reg $1
+	NOTE_CR3_SWITCH \scratch_reg 1
 	mov	%cr3, \scratch_reg
 	ADJUST_KERNEL_CR3 \scratch_reg
 	mov	\scratch_reg, %cr3
@@ -197,7 +197,7 @@ For 32-bit we have the following conventions - kernel is built with
 	PER_CPU_VAR(cpu_tlbstate + TLB_STATE_user_pcid_flush_mask)
 
 .macro SWITCH_TO_USER_CR3 scratch_reg:req scratch_reg2:req
-	NOTE_CR3_SWITCH \scratch_reg $0
+	NOTE_CR3_SWITCH \scratch_reg 0
 	mov	%cr3, \scratch_reg
 
 	ALTERNATIVE "jmp .Lwrcr3_\@", "", X86_FEATURE_PCID
@@ -245,7 +245,7 @@ For 32-bit we have the following conventions - kernel is built with
 
 .macro SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg:req save_reg:req
 	ALTERNATIVE "jmp .Ldone_\@", "", X86_FEATURE_PTI
-	NOTE_CR3_SWITCH \scratch_reg $1
+	NOTE_CR3_SWITCH \scratch_reg 1
 	movq	%cr3, \scratch_reg
 	movq	\scratch_reg, \save_reg
 	/*
@@ -274,7 +274,7 @@ For 32-bit we have the following conventions - kernel is built with
 	bt	$PTI_USER_PGTABLE_BIT, \save_reg
 	jnc	.Lend_\@
 
-	NOTE_CR3_SWITCH \scratch_reg $0
+	NOTE_CR3_SWITCH \scratch_reg 0
 	ALTERNATIVE "jmp .Lwrcr3_\@", "", X86_FEATURE_PCID
 
 	/*