[PATCH] arm64: translate register values to physical addresses in kernel panics

[PATCH] arm64: translate register values to physical addresses in kernel panics

[Peter Collingbourne]

When debugging a kernel panic it is sometimes useful to know the physical
address of any virtual addresses stored in registers. Therefore, pass
all register values through AT S1E1R and print the resulting PAR_EL1
value next to the register.

Signed-off-by: Peter Collingbourne <pcc@google.com>
Link: https://linux-review.googlesource.com/id/I7c6be65f27052da57088ff58b463fdbe2394f43c
---
Not sure if this should land in this form (I imagine there could be
all kinds of parsers that are expecting the existing format) but
maybe behind an option. Let me know what you think.

 arch/arm64/kernel/process.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 92bcc1768f0b..8b5f8c2c8abf 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -197,6 +197,18 @@ static void print_pstate(struct pt_regs *regs)
 	}
 }
 
+static unsigned long at(unsigned long addr)
+{
+	unsigned long pa;
+
+	__asm__ __volatile__("at s1e1r, %1\n"
+			     "mrs %0, par_el1\n"
+			     : "=r"(pa)
+			     : "r"(addr)
+			     : "memory");
+	return pa;
+}
+
 void __show_regs(struct pt_regs *regs)
 {
 	int i, top_reg;
@@ -231,10 +243,10 @@ void __show_regs(struct pt_regs *regs)
 	i = top_reg;
 
 	while (i >= 0) {
-		printk("x%-2d: %016llx", i, regs->regs[i]);
+		printk("x%-2d: %016llx (%016llx)", i, regs->regs[i], at(regs->regs[i]));
 
 		while (i-- % 3)
-			pr_cont(" x%-2d: %016llx", i, regs->regs[i]);
+			pr_cont(" x%-2d: %016llx (%016llx)", i, regs->regs[i], at(regs->regs[i]));
 
 		pr_cont("\n");
 	}
-- 
2.37.1.595.g718a3a8f04-goog


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Re: [PATCH] arm64: translate register values to physical addresses in kernel panics

[Catalin Marinas]

On Fri, Aug 12, 2022 at 11:35:30AM -0700, Peter Collingbourne wrote:
> When debugging a kernel panic it is sometimes useful to know the physical
> address of any virtual addresses stored in registers. Therefore, pass
> all register values through AT S1E1R and print the resulting PAR_EL1
> value next to the register.

I don't see much value in this but I haven't come across a use-case yet.
For page faults the kernel prints the content of the PTE and that's what
I'm usually interested in.

> Not sure if this should land in this form (I imagine there could be
> all kinds of parsers that are expecting the existing format) but
> maybe behind an option. Let me know what you think.

While that's not considered user ABI, there might be some scripts
parsing it, though I suspect they don't pay attention to the registers
(I might be wrong though).

> +static unsigned long at(unsigned long addr)
> +{
> +	unsigned long pa;
> +
> +	__asm__ __volatile__("at s1e1r, %1\n"
> +			     "mrs %0, par_el1\n"
> +			     : "=r"(pa)
> +			     : "r"(addr)
> +			     : "memory");
> +	return pa;
> +}

This should take the translation fault into account. If PAR_EL1.F is 1,
the other bits can't be treated as a physical address. Also if you want
the actual address, it's also worth masking out the non-relevant bits
from PAR_EL1 and adding the offset from 'addr' into the lower 12 bits.

>  void __show_regs(struct pt_regs *regs)
>  {
>  	int i, top_reg;
> @@ -231,10 +243,10 @@ void __show_regs(struct pt_regs *regs)
>  	i = top_reg;
>  
>  	while (i >= 0) {
> -		printk("x%-2d: %016llx", i, regs->regs[i]);
> +		printk("x%-2d: %016llx (%016llx)", i, regs->regs[i], at(regs->regs[i]));
>  
>  		while (i-- % 3)
> -			pr_cont(" x%-2d: %016llx", i, regs->regs[i]);
> +			pr_cont(" x%-2d: %016llx (%016llx)", i, regs->regs[i], at(regs->regs[i]));

How long are the lines printed here? Maybe a better option without
cluttering the register values is to do another pass through the
register and print the potential VA->PA translations (only those kernel
addresses that do not fault). If one is interested they could look them
up on the following lines.

-- 
Catalin

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