linux-kernel - Re: [PATCH 1/3] x86/boot: Fix page table access in 5-level to 4-level paging transition

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Message-ID: <8283c1b6-1487-49e2-b220-7dbd043a2913@intel.com>
Date: Wed, 22 Oct 2025 16:16:34 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Usama Arif <usamaarif642@...il.com>, dwmw@...zon.co.uk,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, ardb@...nel.org, hpa@...or.com
Cc: x86@...nel.org, apopple@...dia.com, thuth@...hat.com,
 nik.borisov@...e.com, kas@...nel.org, linux-kernel@...r.kernel.org,
 linux-efi@...r.kernel.org, kernel-team@...a.com,
 Michael van der Westhuizen <rmikey@...a.com>, Tobias Fleig <tfleig@...a.com>
Subject: Re: [PATCH 1/3] x86/boot: Fix page table access in 5-level to 4-level
 paging transition

On 10/22/25 15:06, Usama Arif wrote:
> +		pgdp = (pgd_t *)read_cr3_pa();
> +		new_cr3 = (u64 *)(pgd_val(pgdp[0]) & PTE_PFN_MASK);
> +		memcpy(trampoline_32bit, new_cr3, PAGE_SIZE);

Heh, somebody like casting, I see!

But seriously, read_cr3_pa() should be returning a physical address. No?
Today it does:

static inline unsigned long read_cr3_pa(void)
{
        return __read_cr3() & CR3_ADDR_MASK;
}

So shouldn't CR3_ADDR_MASK be masking out any naughty non-address bits?
Shouldn't we fix read_cr3_pa() and not do this in its caller?