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Message-ID: <63297d29-bb24-ac5e-0b47-35e22bb1a2f8@amd.com>
Date: Fri, 12 Jan 2024 14:28:13 -0600
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Subject: Re: [PATCH v1 11/26] x86/sev: Invalidate pages from the direct map
 when adding them to the RMP table
To: Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@intel.com>
Cc: Michael Roth <michael.roth@amd.com>, x86@kernel.org, kvm@vger.kernel.org,
 linux-coco@lists.linux.dev, linux-mm@kvack.org,
 linux-crypto@vger.kernel.org, linux-kernel@vger.kernel.org,
 tglx@linutronix.de, mingo@redhat.com, jroedel@suse.de, hpa@zytor.com,
 ardb@kernel.org, pbonzini@redhat.com, seanjc@google.com,
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 dave.hansen@linux.intel.com, slp@redhat.com, pgonda@google.com,
 peterz@infradead.org, srinivas.pandruvada@linux.intel.com,
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 ak@linux.intel.com, tony.luck@intel.com,
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 jarkko@kernel.org, ashish.kalra@amd.com, nikunj.dadhania@amd.com,
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 Brijesh Singh <brijesh.singh@amd.com>
References: <20231230161954.569267-1-michael.roth@amd.com>
 <20231230161954.569267-12-michael.roth@amd.com>
 <cb604c37-aeb5-45bd-b6db-246ae724e4ca@intel.com>
 <20240112200751.GHZaGcF0-OZVJiIB7y@fat_crate.local>
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From: Tom Lendacky <thomas.lendacky@amd.com>
In-Reply-To: <20240112200751.GHZaGcF0-OZVJiIB7y@fat_crate.local>
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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...
>