Message-ID: <fc357e47-a727-4ba3-9019-877ce8bc443f@arm.com>
Date: Wed, 26 Feb 2025 17:15:23 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Mark Rutland <mark.rutland@....com>, Ryan Roberts <ryan.roberts@....com>
Cc: linux-arm-kernel@...ts.infradead.org,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: Explicit cast conversions to correct data type



On 2/25/25 19:22, Mark Rutland wrote:
> On Tue, Feb 25, 2025 at 01:00:40PM +0000, Ryan Roberts wrote:
>> On 25/02/2025 12:32, Mark Rutland wrote:
>>> On Wed, Feb 19, 2025 at 09:26:46AM +0530, Anshuman Khandual wrote:
>>>> From: Ryan Roberts <ryan.roberts@....com>
>>>>
>>>> When CONFIG_ARM64_PA_BITS_52 is enabled, page table helpers __pte_to_phys()
>>>> and __phys_to_pte_val() are functions which return phys_addr_t and pteval_t
>>>> respectively as expected. But otherwise without this config being enabled,
>>>> they are defined as macros and their return types are implicit.
>>>>
>>>> Until now this has worked out correctly as both pte_t and phys_addr_t data
>>>> types have been 64 bits. But with the introduction of 128 bit page tables,
>>>> pte_t becomes 128 bits. Hence this ends up with incorrect widths after the
>>>> conversions, which leads to compiler warnings.
>>>
>>> Does 128-bit page table not imply 52-bit PAs?
>>
>> Not to my knowledge. For now the prototype code base is explicitly sticking to
>> 48-bit PA and 44-bit VA (for initial simplicitly because that's the limit for 4
>> levels).
> 
> Fair enough; info dump below, but hopefully nothing of consequence.
> 
> I assume that you're relying on the VMSAv9-128 PA bits [48:12] being in the
> same place as in the VMSAv8-64 descriptors, and being handled by the same
> PTE_ADDR_LOW mask that we use for CONFIG_ARM64_PA_BITS_52=n.
> 
>>>From a quick scan of ARM DDI 0487 L.a, the VMSAv9-128 translation table
> descriptor format always contains a 56-bit PA (though PARange could be
> smaller than that). Bits [51:49] are packed differently than in
> VMSAv8-64 descriptors, and bits [55:52] are obviously new.
> 
>>>> Fix the warnings by explicitly casting to the correct type after doing the
>>>> conversion.
>>>
>>> I think it would be simpler and clearer if we replaced the macros with
>>> functions, such that __pte_to_phys() and __phys_to_pte_val() are
>>> *always* functions.
>>
>> Yeah, agreed. This was initially just a hack I did to get things working.
> 
> Cool; sounds like we're aligned.

Planning for the following respin instead.

diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
index 0b2a2ad1b9e8..4ebfa60ea5c6 100644
--- a/arch/arm64/include/asm/pgtable.h
+++ b/arch/arm64/include/asm/pgtable.h
@@ -68,10 +68,6 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
 #define pte_ERROR(e)	\
 	pr_err("%s:%d: bad pte %016llx.\n", __FILE__, __LINE__, pte_val(e))
 
-/*
- * Macros to convert between a physical address and its placement in a
- * page table entry, taking care of 52-bit addresses.
- */
 #ifdef CONFIG_ARM64_PA_BITS_52
 static inline phys_addr_t __pte_to_phys(pte_t pte)
 {
@@ -84,8 +80,15 @@ static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
 	return (phys | (phys >> PTE_ADDR_HIGH_SHIFT)) & PHYS_TO_PTE_ADDR_MASK;
 }
 #else
-#define __pte_to_phys(pte)	(pte_val(pte) & PTE_ADDR_LOW)
-#define __phys_to_pte_val(phys)	(phys)
+static inline phys_addr_t __pte_to_phys(pte_t pte)
+{
+	return pte_val(pte) & PTE_ADDR_LOW;
+}
+
+static inline pteval_t __phys_to_pte_val(phys_addr_t phys)
+{
+	return phys;
+}
 #endif
 
 #define pte_pfn(pte)		(__pte_to_phys(pte) >> PAGE_SHIFT)
-- 
2.30.2

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