linux-kernel - Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map when adding them to the RMP table

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Date: Fri, 12 Jan 2024 14:28:13 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...el.com>
Cc: Michael Roth <michael.roth@....com>, x86@...nel.org, kvm@...r.kernel.org,
 linux-coco@...ts.linux.dev, linux-mm@...ck.org,
 linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
 tglx@...utronix.de, mingo@...hat.com, jroedel@...e.de, hpa@...or.com,
 ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
 vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org,
 dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com,
 peterz@...radead.org, srinivas.pandruvada@...ux.intel.com,
 rientjes@...gle.com, tobin@....com, vbabka@...e.cz, kirill@...temov.name,
 ak@...ux.intel.com, tony.luck@...el.com,
 sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
 jarkko@...nel.org, ashish.kalra@....com, nikunj.dadhania@....com,
 pankaj.gupta@....com, liam.merwick@...cle.com, zhi.a.wang@...el.com,
 Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map
 when adding them to the RMP table



On 1/12/24 14:07, Borislav Petkov wrote:
> On Fri, Jan 12, 2024 at 12:00:01PM -0800, Dave Hansen wrote:
>> On 12/30/23 08:19, Michael Roth wrote:
>>> If the kernel uses a 2MB directmap mapping to write to an address, and
>>> that 2MB range happens to contain a 4KB page that set to private in the
>>> RMP table, that will also lead to a page-fault exception.

I thought there was also a desire to remove the direct map for any pages 
assigned to a guest as private, not just the case that the comment says. 
So updating the comment would probably the best action.

>>
>> I thought we agreed long ago to just demote the whole direct map to 4k
>> on kernels that might need to act as SEV-SNP hosts.  That should be step
>> one and this can be discussed as an optimization later.

Won't this accomplish that without actually demoting a lot of long-live 
kernel related mappings that would never be demoted? I don't think we need 
to demote the whole mapping to 4K.

Thanks,
Tom

> 
> What would be the disadvantage here? Higher TLB pressure when running
> kernel code I guess...
>