linux-kernel - Re: [RFC V1 5/5] x86: CVMs: Ensure that memory conversions happen at 2M alignment

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Message-ID: <CAGtprH_ANUVU+Dh1KOq0vpT7BGbCEvD2ab9B=sxjzHYsKxFGeA@mail.gmail.com>
Date: Thu, 1 Feb 2024 09:16:03 +0530
From: Vishal Annapurve <vannapurve@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, pbonzini@...hat.com, 
	rientjes@...gle.com, seanjc@...gle.com, erdemaktas@...gle.com, 
	ackerleytng@...gle.com, jxgao@...gle.com, sagis@...gle.com, oupton@...gle.com, 
	peterx@...hat.com, vkuznets@...hat.com, dmatlack@...gle.com, 
	pgonda@...gle.com, michael.roth@....com, kirill@...temov.name, 
	thomas.lendacky@....com, dave.hansen@...ux.intel.com, 
	linux-coco@...ts.linux.dev, chao.p.peng@...ux.intel.com, 
	isaku.yamahata@...il.com, andrew.jones@...ux.dev, corbet@....net, hch@....de, 
	m.szyprowski@...sung.com, rostedt@...dmis.org, iommu@...ts.linux.dev
Subject: Re: [RFC V1 5/5] x86: CVMs: Ensure that memory conversions happen at
 2M alignment

On Wed, Jan 31, 2024 at 10:03 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 1/11/24 21:52, Vishal Annapurve wrote:
> > @@ -2133,8 +2133,10 @@ static int __set_memory_enc_pgtable(unsigned long addr, int numpages, bool enc)
> >       int ret;
> >
> >       /* Should not be working on unaligned addresses */
> > -     if (WARN_ONCE(addr & ~PAGE_MASK, "misaligned address: %#lx\n", addr))
> > -             addr &= PAGE_MASK;
> > +     if (WARN_ONCE(addr & ~HPAGE_MASK, "misaligned address: %#lx\n", addr)
> > +             || WARN_ONCE((numpages << PAGE_SHIFT) & ~HPAGE_MASK,
> > +                     "misaligned numpages: %#lx\n", numpages))
> > +             return -EINVAL;
>
> This series is talking about swiotlb and DMA, then this applies a
> restriction to what I *thought* was a much more generic function:
> __set_memory_enc_pgtable().  What prevents this function from getting
> used on 4k mappings?
>
>

The end goal here is to limit the conversion granularity to hugepage
sizes. SWIOTLB allocations are the major source of unaligned
allocations(and so the conversions) that need to be fixed before
achieving this goal.

This change will ensure that conversion fails for unaligned ranges, as
I don't foresee the need for 4K aligned conversions apart from DMA
allocations.