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From: Sean Christopherson <seanjc@google.com>
To: Brijesh Singh <brijesh.singh@amd.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, linux-efi@vger.kernel.org,
	platform-driver-x86@vger.kernel.org, linux-coco@lists.linux.dev,
	linux-mm@kvack.org, linux-crypto@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Sergio Lopez <slp@redhat.com>, Peter Gonda <pgonda@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	David Rientjes <rientjes@google.com>,
	Dov Murik <dovmurik@linux.ibm.com>,
	Tobin Feldman-Fitzthum <tobin@ibm.com>,
	Borislav Petkov <bp@alien8.de>,
	Michael Roth <michael.roth@amd.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	tony.luck@intel.com, npmccallum@redhat.com,
	brijesh.ksingh@gmail.com
Subject: Re: [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when adding the page in RMP table
Date: Wed, 14 Jul 2021 22:25:03 +0000	[thread overview]
Message-ID: <YO9kP1v0TAFXISHD@google.com> (raw)
In-Reply-To: <20210707183616.5620-8-brijesh.singh@amd.com>

On Wed, Jul 07, 2021, Brijesh Singh wrote:
> The integrity guarantee of SEV-SNP is enforced through the RMP table.
> The RMP is used in conjuntion with standard x86 and IOMMU page
> tables to enforce memory restrictions and page access rights. The
> RMP is indexed by system physical address, and is checked at the end
> of CPU and IOMMU table walks. The RMP check is enforced as soon as
> SEV-SNP is enabled globally in the system. Not every memory access
> requires an RMP check. In particular, the read accesses from the
> hypervisor do not require RMP checks because the data confidentiality
> is already protected via memory encryption. When hardware encounters
> an RMP checks failure, it raise a page-fault exception. The RMP bit in
> fault error code can be used to determine if the fault was due to an
> RMP checks failure.
> 
> 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. If one of entry does not have assigned bit 0 then hardware
> will raise an RMP violation. To resolve it, split the page table entry
> leading to target page into 4K.

Isn't the above just saying:

  All RMP entries covered by a large page must match the shared vs. encrypted
  state of the page, e.g. host large pages must have assigned=0 for all relevant
  RMP entries.

> 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. 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.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
>  arch/x86/kernel/sev.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 949efe530319..a482e01f880a 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -2375,6 +2375,12 @@ int rmpupdate(struct page *page, struct rmpupdate *val)
>  	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
>  		return -ENXIO;
>  
> +	ret = set_memory_4k((unsigned long)page_to_virt(page), 1);

IIUC, this shatters the direct map for page that's assigned to an SNP guest, and
the large pages are never recovered?

I believe a better approach would be to do something similar to memfd_secret[*],
which encountered a similar problem with the direct map.  Instead of forcing the
direct map to be forever 4k, unmap the direct map when making a page guest private,
and restore the direct map when it's made shared (or freed).

I thought memfd_secret had also solved the problem of restoring large pages in
the direct map, but at a glance I can't tell if that's actually implemented
anywhere.  But, even if it's not currently implemented, I think it makes sense
to mimic the memfd_secret approach so that both features can benefit if large
page preservation/restoration is ever added.

[*] https://lkml.kernel.org/r/20210518072034.31572-5-rppt@kernel.org

> +	if (ret) {
> +		pr_err("Failed to split physical address 0x%lx (%d)\n", spa, ret);
> +		return ret;
> +	}
> +
>  	/* Retry if another processor is modifying the RMP entry. */
>  	do {
>  		/* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
> -- 
> 2.17.1
> 

  reply	other threads:[~2021-07-14 22:25 UTC|newest]

Thread overview: 176+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-07 18:35 [PATCH Part2 RFC v4 00/40] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 01/40] KVM: SVM: Add support to handle AP reset MSR protocol Brijesh Singh
2021-07-14 20:17   ` Sean Christopherson
2021-07-15  7:39     ` Joerg Roedel
2021-07-15 13:42     ` Tom Lendacky
2021-07-15 15:45       ` Sean Christopherson
2021-07-15 17:05         ` Tom Lendacky
2021-07-07 18:35 ` [PATCH Part2 RFC v4 02/40] KVM: SVM: Provide the Hypervisor Feature support VMGEXIT Brijesh Singh
2021-07-14 20:37   ` Sean Christopherson
2021-07-14 21:00     ` Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 03/40] x86/cpufeatures: Add SEV-SNP CPU feature Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 04/40] x86/sev: Add the host SEV-SNP initialization support Brijesh Singh
2021-07-14 21:07   ` Sean Christopherson
2021-07-14 22:02     ` Brijesh Singh
2021-07-14 22:06       ` Sean Christopherson
2021-07-14 22:11         ` Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 05/40] x86/sev: Add RMP entry lookup helpers Brijesh Singh
2021-07-15 18:37   ` Sean Christopherson
2021-07-15 19:28     ` Brijesh Singh
2021-07-16 17:22       ` Brijesh Singh
2021-07-20 22:06         ` Sean Christopherson
2021-07-20 23:10           ` Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 06/40] x86/sev: Add helper functions for RMPUPDATE and PSMASH instruction Brijesh Singh
2021-07-12 18:44   ` Peter Gonda
2021-07-12 19:00     ` Dave Hansen
2021-07-15 18:56       ` Sean Christopherson
2021-07-15 19:08         ` Dave Hansen
2021-07-15 19:18           ` Sean Christopherson
2021-07-07 18:35 ` [PATCH Part2 RFC v4 07/40] x86/sev: Split the physmap when adding the page in RMP table Brijesh Singh
2021-07-14 22:25   ` Sean Christopherson [this message]
2021-07-15 17:05     ` Brijesh Singh
2021-07-15 17:51       ` Sean Christopherson
2021-07-15 18:14         ` Brijesh Singh
2021-07-15 18:39           ` Sean Christopherson
2021-07-15 19:38             ` Brijesh Singh
2021-07-15 22:01               ` Sean Christopherson
2021-07-15 22:11                 ` Brijesh Singh
2021-07-30 11:31               ` Vlastimil Babka
2021-07-30 16:10                 ` Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 08/40] x86/traps: Define RMP violation #PF error code Brijesh Singh
2021-07-15 19:02   ` Sean Christopherson
2021-07-15 19:16     ` Dave Hansen
2021-07-07 18:35 ` [PATCH Part2 RFC v4 09/40] x86/fault: Add support to dump RMP entry on fault Brijesh Singh
2021-07-07 19:21   ` Dave Hansen
2021-07-08 15:02     ` Brijesh Singh
2021-07-08 15:30       ` Dave Hansen
2021-07-08 16:48         ` Brijesh Singh
2021-07-08 16:58           ` Dave Hansen
2021-07-08 17:11             ` Brijesh Singh
2021-07-08 17:15               ` Dave Hansen
2021-07-07 18:35 ` [PATCH Part2 RFC v4 10/40] x86/fault: Add support to handle the RMP fault for user address Brijesh Singh
2021-07-08 16:16   ` Dave Hansen
2021-07-12 15:43     ` Brijesh Singh
2021-07-12 16:00       ` Dave Hansen
2021-07-12 16:11         ` Brijesh Singh
2021-07-12 16:15           ` Dave Hansen
2021-07-12 16:24             ` Brijesh Singh
2021-07-12 16:29               ` Dave Hansen
2021-07-12 16:49                 ` Brijesh Singh
2021-07-15 21:53                   ` Sean Christopherson
2021-07-30 16:00   ` Vlastimil Babka
2021-07-30 16:31     ` Dave Hansen
2021-07-07 18:35 ` [PATCH Part2 RFC v4 11/40] crypto:ccp: Define the SEV-SNP commands Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 12/40] crypto: ccp: Add support to initialize the AMD-SP for SEV-SNP Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 13/40] crypto: ccp: Shutdown SNP firmware on kexec Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 14/40] crypto:ccp: Provide APIs to issue SEV-SNP commands Brijesh Singh
2021-07-08 18:56   ` Dr. David Alan Gilbert
2021-07-07 18:35 ` [PATCH Part2 RFC v4 15/40] crypto: ccp: Handle the legacy TMR allocation when SNP is enabled Brijesh Singh
2021-07-14 13:22   ` Marc Orr
2021-07-14 16:45     ` Brijesh Singh
2021-07-14 18:14       ` Marc Orr
2021-07-15 23:48   ` Sean Christopherson
2021-07-16 12:55     ` Brijesh Singh
2021-07-16 15:35       ` Sean Christopherson
2021-07-16 15:47         ` Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 16/40] crypto: ccp: Handle the legacy SEV command " Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 17/40] crypto: ccp: Add the SNP_PLATFORM_STATUS command Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 18/40] crypto: ccp: Add the SNP_{SET,GET}_EXT_CONFIG command Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 19/40] crypto: ccp: provide APIs to query extended attestation report Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 20/40] KVM: SVM: Make AVIC backing, VMSA and VMCB memory allocation SNP safe Brijesh Singh
2021-07-14 13:35   ` Marc Orr
2021-07-14 16:47     ` Brijesh Singh
2021-07-20 18:02   ` Sean Christopherson
2021-08-03 14:38     ` Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 21/40] KVM: SVM: Add initial SEV-SNP support Brijesh Singh
2021-07-16 18:00   ` Sean Christopherson
2021-07-16 18:46     ` Brijesh Singh
2021-07-16 19:31       ` Sean Christopherson
2021-07-16 21:03         ` Brijesh Singh
2021-07-07 18:35 ` [PATCH Part2 RFC v4 22/40] KVM: SVM: Add KVM_SNP_INIT command Brijesh Singh
2021-07-16 19:33   ` Sean Christopherson
2021-07-16 21:25     ` Brijesh Singh
2021-07-19 20:24       ` Sean Christopherson
2021-07-07 18:35 ` [PATCH Part2 RFC v4 23/40] KVM: SVM: Add KVM_SEV_SNP_LAUNCH_START command Brijesh Singh
2021-07-12 18:45   ` Peter Gonda
2021-07-16 19:43   ` Sean Christopherson
2021-07-16 21:42     ` Brijesh Singh
2021-07-07 18:36 ` [PATCH Part2 RFC v4 24/40] KVM: SVM: Add KVM_SEV_SNP_LAUNCH_UPDATE command Brijesh Singh
2021-07-16 20:01   ` Sean Christopherson
2021-07-16 22:00     ` Brijesh Singh
2021-07-19 20:51       ` Sean Christopherson
2021-07-19 21:34         ` Brijesh Singh
2021-07-19 21:36           ` Brijesh Singh
2021-07-07 18:36 ` [PATCH Part2 RFC v4 25/40] KVM: SVM: Reclaim the guest pages when SEV-SNP VM terminates Brijesh Singh
2021-07-16 20:09   ` Sean Christopherson
2021-07-16 22:16     ` Brijesh Singh
2021-07-17  0:46       ` Sean Christopherson
2021-07-19 12:55         ` Brijesh Singh
2021-07-19 17:18           ` Sean Christopherson
2021-07-19 18:34             ` Brijesh Singh
2021-07-19 19:03               ` Sean Christopherson
2021-07-19 19:14                 ` Sean Christopherson
2021-07-19 19:37                 ` Brijesh Singh
2021-07-20 16:40                   ` Sean Christopherson
2021-07-20 18:23                     ` Brijesh Singh
2021-07-07 18:36 ` [PATCH Part2 RFC v4 26/40] KVM: SVM: Add KVM_SEV_SNP_LAUNCH_FINISH command Brijesh Singh
2021-07-16 20:18   ` Sean Christopherson
2021-07-16 22:48     ` Brijesh Singh
2021-07-19 16:54       ` Sean Christopherson
2021-07-19 18:29         ` Brijesh Singh
2021-07-19 19:14           ` Sean Christopherson
2021-07-19 19:49             ` Brijesh Singh
2021-07-19 20:13               ` Sean Christopherson
2021-07-21 17:53         ` Marc Orr
2021-07-07 18:36 ` [PATCH Part2 RFC v4 27/40] KVM: X86: Add kvm_x86_ops to get the max page level for the TDP Brijesh Singh
2021-07-16 19:19   ` Sean Christopherson
2021-07-16 20:41     ` Brijesh Singh
2021-07-20 19:38       ` Sean Christopherson
2021-07-20 20:06         ` Brijesh Singh
2021-07-07 18:36 ` [PATCH Part2 RFC v4 28/40] KVM: X86: Introduce kvm_mmu_map_tdp_page() for use by SEV Brijesh Singh
2021-07-16 18:15   ` Sean Christopherson
2021-07-07 18:36 ` [PATCH Part2 RFC v4 29/40] KVM: X86: Introduce kvm_mmu_get_tdp_walk() for SEV-SNP use Brijesh Singh
2021-07-07 18:36 ` [PATCH Part2 RFC v4 30/40] KVM: X86: Define new RMP check related #NPF error bits Brijesh Singh
2021-07-16 20:22   ` Sean Christopherson
2021-07-17  0:34     ` Brijesh Singh
2021-07-07 18:36 ` [PATCH Part2 RFC v4 31/40] KVM: X86: update page-fault trace to log the 64-bit error code Brijesh Singh
2021-07-16 20:25   ` Sean Christopherson
2021-07-17  0:35     ` Brijesh Singh
2021-07-07 18:36 ` [PATCH Part2 RFC v4 32/40] KVM: SVM: Add support to handle GHCB GPA register VMGEXIT Brijesh Singh
2021-07-16 20:45   ` Sean Christopherson
2021-07-17  0:44     ` Brijesh Singh
2021-07-19 20:04       ` Sean Christopherson
2021-07-07 18:36 ` [PATCH Part2 RFC v4 33/40] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT Brijesh Singh
2021-07-16 21:00   ` Sean Christopherson
2021-07-19 14:19     ` Brijesh Singh
2021-07-19 18:55       ` Sean Christopherson
2021-07-19 19:15         ` Brijesh Singh
2021-08-13 16:32         ` Borislav Petkov
2021-07-07 18:36 ` [PATCH Part2 RFC v4 34/40] KVM: SVM: Add support to handle " Brijesh Singh
2021-07-16 21:14   ` Sean Christopherson
2021-07-19 14:24     ` Brijesh Singh
2021-07-07 18:36 ` [PATCH Part2 RFC v4 35/40] KVM: Add arch hooks to track the host write to guest memory Brijesh Singh
2021-07-19 23:30   ` Sean Christopherson
2021-07-20 15:15     ` Brijesh Singh
2021-07-07 18:36 ` [PATCH Part2 RFC v4 36/40] KVM: X86: Export the kvm_zap_gfn_range() for the SNP use Brijesh Singh
2021-07-07 18:36 ` [PATCH Part2 RFC v4 37/40] KVM: SVM: Add support to handle the RMP nested page fault Brijesh Singh
2021-07-20  0:10   ` Sean Christopherson
2021-07-20 17:55     ` Brijesh Singh
2021-07-20 22:31       ` Sean Christopherson
2021-07-20 23:53         ` Brijesh Singh
2021-07-21 20:15           ` Sean Christopherson
2021-07-07 18:36 ` [PATCH Part2 RFC v4 38/40] KVM: SVM: Provide support for SNP_GUEST_REQUEST NAE event Brijesh Singh
2021-07-19 22:50   ` Sean Christopherson
2021-07-20 14:37     ` Brijesh Singh
2021-07-20 16:28       ` Sean Christopherson
2021-07-20 18:21         ` Brijesh Singh
2021-07-20 22:09           ` Sean Christopherson
2021-07-07 18:36 ` [PATCH Part2 RFC v4 39/40] KVM: SVM: Use a VMSA physical address variable for populating VMCB Brijesh Singh
2021-07-21  0:20   ` Sean Christopherson
2021-07-21 16:26     ` Tom Lendacky
2021-07-07 18:36 ` [PATCH Part2 RFC v4 40/40] KVM: SVM: Support SEV-SNP AP Creation NAE event Brijesh Singh
2021-07-21  0:01   ` Sean Christopherson
2021-07-21 17:47     ` Tom Lendacky
2021-07-21 19:52       ` Sean Christopherson
2021-08-20 14:44         ` Tom Lendacky
2021-07-08 15:40 ` [PATCH Part2 RFC v4 00/40] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support Dave Hansen

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