linux-kernel - Re: [PATCH v5 10/19] arm64: Override set_fixmap

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Message-ID: <a0c8014a-48c1-44c6-a45e-75ca778a4642@arm.com>
Date: Fri, 30 Aug 2024 16:54:45 +0100
From: Steven Price <steven.price@....com>
To: Suzuki K Poulose <suzuki.poulose@....com>, kvm@...r.kernel.org,
 kvmarm@...ts.linux.dev
Cc: Catalin Marinas <catalin.marinas@....com>, Marc Zyngier <maz@...nel.org>,
 Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
 Oliver Upton <oliver.upton@...ux.dev>, Zenghui Yu <yuzenghui@...wei.com>,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Joey Gouly <joey.gouly@....com>, Alexandru Elisei
 <alexandru.elisei@....com>, Christoffer Dall <christoffer.dall@....com>,
 Fuad Tabba <tabba@...gle.com>, linux-coco@...ts.linux.dev,
 Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
 Gavin Shan <gshan@...hat.com>, Shanker Donthineni <sdonthineni@...dia.com>,
 Alper Gun <alpergun@...gle.com>
Subject: Re: [PATCH v5 10/19] arm64: Override set_fixmap_io

On 19/08/2024 15:13, Suzuki K Poulose wrote:
> On 19/08/2024 14:19, Steven Price wrote:
>> From: Suzuki K Poulose <suzuki.poulose@....com>
>>
>> Override the set_fixmap_io to set shared permission for the host
>> in case of a CC guest. For now we mark it shared unconditionally.
>>
>> If/when support for device assignment and device emulation in the realm
>> is added in the future then this will need to filter the physical
>> address and make the decision accordingly.
>>
>> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
>> Signed-off-by: Steven Price <steven.price@....com>
>> ---
>> New patch for v5
>> ---
>>   arch/arm64/include/asm/fixmap.h |  2 ++
>>   arch/arm64/mm/mmu.c             | 17 +++++++++++++++++
>>   2 files changed, 19 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/fixmap.h
>> b/arch/arm64/include/asm/fixmap.h
>> index 87e307804b99..2c20da3a468c 100644
>> --- a/arch/arm64/include/asm/fixmap.h
>> +++ b/arch/arm64/include/asm/fixmap.h
>> @@ -108,6 +108,8 @@ void __init early_fixmap_init(void);
>>   #define __late_clear_fixmap(idx) __set_fixmap((idx), 0,
>> FIXMAP_PAGE_CLEAR)
>>     extern void __set_fixmap(enum fixed_addresses idx, phys_addr_t
>> phys, pgprot_t prot);
>> +#define set_fixmap_io set_fixmap_io
>> +void set_fixmap_io(enum fixed_addresses idx, phys_addr_t phys);
>>     #include <asm-generic/fixmap.h>
>>   diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 353ea5dc32b8..06b66c23c124 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -1193,6 +1193,23 @@ void vmemmap_free(unsigned long start, unsigned
>> long end,
>>   }
>>   #endif /* CONFIG_MEMORY_HOTPLUG */
>>   +void set_fixmap_io(enum fixed_addresses idx, phys_addr_t phys)
>> +{
>> +    pgprot_t prot = FIXMAP_PAGE_IO;
>> +
>> +    /*
>> +     * The set_fixmap_io maps a single Page covering phys.
>> +     * To make better decision, we stick to the smallest page
>> +     * size supported (4K).
>> +     */
>> +    if (!arm64_is_iomem_private(phys, SZ_4K))
>> +        prot = pgprot_decrypted(prot);
>> +    else
>> +        prot = pgprot_encrypted(prot);
> 
> With the ioremap_prot_hook introduction, this one looks like should use
> that, instead of open coding the same thing. Thoughts ?

I can see the logic, but it's not quite that simple to implement. Either
the ioremap_prot_hook needs moving out (and presumably) renaming to be a
more generic "override device pgprot" mechanism, we it would be a horrid
hack to put ioremap code in arch/arm64/mm/ioremap.c to handle this.

Given that we're not even sure that the set_fixmap hack is a good idea,
I'll leave this alone for now - the open coding is clear even if it's a
duplicate.

Thanks,
Steve