Re: [RFC Part2 PATCH 04/30] x86/mm: split the physmap when adding the page in RMP table

From: Borislav Petkov <bp@alien8.de>
To: Brijesh Singh <brijesh.singh@amd.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	kvm@vger.kernel.org, linux-crypto@vger.kernel.org,
	ak@linux.intel.com, herbert@gondor.apana.org.au,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Tony Luck <tony.luck@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	David Rientjes <rientjes@google.com>,
	Sean Christopherson <seanjc@google.com>,
	Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [RFC Part2 PATCH 04/30] x86/mm: split the physmap when adding the page in RMP table
Date: Mon, 19 Apr 2021 14:32:26 +0200	[thread overview]
Message-ID: <20210419123226.GC9093@zn.tnic> (raw)
In-Reply-To: <20210324170436.31843-5-brijesh.singh@amd.com>

On Wed, Mar 24, 2021 at 12:04:10PM -0500, Brijesh Singh wrote:
> A write from the hypervisor goes through the RMP checks. When the
> hypervisor writes to pages, hardware checks to ensures that the assigned
> bit in the RMP is zero (i.e page is shared). If the page table entry that
> gives the sPA indicates that the target page size is a large page, then
> all RMP entries for the 4KB constituting pages of the target must have the
> assigned bit 0.

Hmm, so this is important: I read this such that we can have a 2M
page table entry but the RMP table can contain 4K entries for the
corresponding 512 4K pages. Is that correct?

If so, then there's a certain discrepancy here and I'd expect that if
the page gets split/collapsed, depending on the result, the RMP table
should be updated too, so that it remains in sync.

For example:

* mm decides to group all 512 4K entries into a 2M entry, RMP table gets
updated in the end to reflect that

* mm decides to split a page, RMP table gets updated too, for the same
reason.

In this way, RMP table will be always in sync with the pagetables.

I know, I probably am missing something but that makes most sense to
me instead of noticing the discrepancy and getting to work then, when
handling the RMP violation.

Or?

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette