linux-kernel - Re: [PATCH 05/11] x86: remove HIGHMEM64G support

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Message-ID: <CAHp75VfyBLTnY1kReQ-ALngWqPoyLaHhsmT1shR_UzpL8sK1UA@mail.gmail.com>
Date: Wed, 4 Dec 2024 20:37:03 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Arnd Bergmann <arnd@...db.de>
Cc: "H. Peter Anvin" <hpa@...or.com>, Brian Gerst <brgerst@...il.com>, Arnd Bergmann <arnd@...nel.org>, 
	linux-kernel@...r.kernel.org, x86@...nel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>, Andy Shevchenko <andy@...nel.org>, 
	Matthew Wilcox <willy@...radead.org>, Sean Christopherson <seanjc@...gle.com>, 
	Davide Ciminaghi <ciminaghi@...dd.com>, Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH 05/11] x86: remove HIGHMEM64G support

On Wed, Dec 4, 2024 at 6:57 PM Arnd Bergmann <arnd@...db.de> wrote:
> On Wed, Dec 4, 2024, at 17:37, H. Peter Anvin wrote:
> > On December 4, 2024 5:29:17 AM PST, Brian Gerst <brgerst@...il.com> wrote:
> >>>
> >>> Removing this also drops the need for PHYS_ADDR_T_64BIT and SWIOTLB.
> >>> PAE mode is still required to get access to the 'NX' bit on Atom
> >>> 'Pentium M' and 'Core Duo' CPUs.
> >
> > By the way, there are 64-bit machines which require swiotlb.
>
> What I meant to write here was that CONFIG_X86_PAE no longer
> needs to select PHYS_ADDR_T_64BIT and SWIOTLB. I ended up
> splitting that change out to patch 06/11 with a better explanation,
> so the sentence above is just wrong now and I've removed it
> in my local copy now.
>
> Obviously 64-bit kernels still generally need swiotlb.

Theoretically swiotlb can be useful on 32-bit machines as well for the
DMA controllers that have < 32-bit mask. Dunno if swiotlb was designed
to run on 32-bit machines at all.


-- 
With Best Regards,
Andy Shevchenko