linux-kernel - Re: [PATCH Part2 RFC v2 08/37] x86/sev: Split the physmap when adding the page in RMP table

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Date:   Mon, 3 May 2021 17:07:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        tglx@...utronix.de, bp@...en8.de, jroedel@...e.de,
        thomas.lendacky@....com, pbonzini@...hat.com, mingo@...hat.com,
        dave.hansen@...el.com, rientjes@...gle.com, seanjc@...gle.com,
        hpa@...or.com, tony.luck@...el.com
Subject: Re: [PATCH Part2 RFC v2 08/37] x86/sev: Split the physmap when
 adding the page in RMP table

On Fri, Apr 30, 2021 at 07:37:53AM -0500, Brijesh Singh wrote:

> This poses a challenge in the Linux memory model. The Linux kernel
> creates a direct mapping of all the physical memory -- referred to as
> the physmap. The physmap may contain a valid mapping of guest owned pages.
> During the page table walk, the host access may get into the situation where
> one of the pages within the large page is owned by the guest (i.e assigned
> bit is set in RMP). A write to a non-guest within the large page will
> raise an RMP violation. To workaround it, call set_memory_4k() to split
> the physmap before adding the page in the RMP table. This ensures that the
> pages added in the RMP table are used as 4K in the physmap.

What's an RMP violation and why are they a problem?

> The spliting of the physmap is a temporary solution until the kernel page
> fault handler is improved to split the kernel address on demand.

How is that an improvement? Fracturing the physmap sucks whichever way
around.

> One of the
> disadvtange of splitting is that eventually, it will end up breaking down
> the entire physmap unless its coalesce back to a large page. I am open to
> the suggestation on various approaches we could take to address this problem.

Have the hardware fracture the TLB entry internally?