From: Sean Christopherson <seanjc@google.com>
To: Michael Roth <michael.roth@amd.com>
Cc: kvm@vger.kernel.org, linux-coco@lists.linux.dev,
linux-mm@kvack.org, linux-crypto@vger.kernel.org,
x86@kernel.org, linux-kernel@vger.kernel.org,
tglx@linutronix.de, mingo@redhat.com, jroedel@suse.de,
thomas.lendacky@amd.com, hpa@zytor.com, ardb@kernel.org,
pbonzini@redhat.com, vkuznets@redhat.com, jmattson@google.com,
luto@kernel.org, dave.hansen@linux.intel.com, slp@redhat.com,
pgonda@google.com, peterz@infradead.org,
srinivas.pandruvada@linux.intel.com, rientjes@google.com,
dovmurik@linux.ibm.com, tobin@ibm.com, bp@alien8.de,
vbabka@suse.cz, kirill@shutemov.name, ak@linux.intel.com,
tony.luck@intel.com, sathyanarayanan.kuppuswamy@linux.intel.com,
alpergun@google.com, jarkko@kernel.org, ashish.kalra@amd.com,
nikunj.dadhania@amd.com, pankaj.gupta@amd.com,
liam.merwick@oracle.com, papaluri@amd.com
Subject: Re: [PATCH v15 22/23] KVM: SEV: Fix return code interpretation for RMP nested page faults
Date: Fri, 10 May 2024 06:58:45 -0700 [thread overview]
Message-ID: <Zj4oFffc7OQivyV-@google.com> (raw)
In-Reply-To: <20240510015822.503071-2-michael.roth@amd.com>
On Thu, May 09, 2024, Michael Roth wrote:
> The intended logic when handling #NPFs with the RMP bit set (31) is to
> first check to see if the #NPF requires a shared<->private transition
> and, if so, to go ahead and let the corresponding KVM_EXIT_MEMORY_FAULT
> get forwarded on to userspace before proceeding with any handling of
> other potential RMP fault conditions like needing to PSMASH the RMP
> entry/etc (which will be done later if the guest still re-faults after
> the KVM_EXIT_MEMORY_FAULT is processed by userspace).
>
> The determination of whether any userspace handling of
> KVM_EXIT_MEMORY_FAULT is needed is done by interpreting the return code
> of kvm_mmu_page_fault(). However, the current code misinterprets the
> return code, expecting 0 to indicate a userspace exit rather than less
> than 0 (-EFAULT). This leads to the following unexpected behavior:
>
> - for KVM_EXIT_MEMORY_FAULTs resulting for implicit shared->private
> conversions, warnings get printed from sev_handle_rmp_fault()
> because it does not expect to be called for GPAs where
> KVM_MEMORY_ATTRIBUTE_PRIVATE is not set. Standard linux guests don't
> generally do this, but it is allowed and should be handled
> similarly to private->shared conversions rather than triggering any
> sort of warnings
>
> - if gmem support for 2MB folios is enabled (via currently out-of-tree
> code), implicit shared<->private conversions will always result in
> a PSMASH being attempted, even if it's not actually needed to
> resolve the RMP fault. This doesn't cause any harm, but results in a
> needless PSMASH and zapping of the sPTE
>
> Resolve these issues by calling sev_handle_rmp_fault() only when
> kvm_mmu_page_fault()'s return code is greater than or equal to 0,
> indicating a KVM_MEMORY_EXIT_FAULT/-EFAULT isn't needed. While here,
> simplify the code slightly and fix up the associated comments for better
> clarity.
>
> Fixes: ccc9d836c5c3 ("KVM: SEV: Add support to handle RMP nested page faults")
>
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> ---
> arch/x86/kvm/svm/svm.c | 10 ++++------
> 1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 426ad49325d7..9431ce74c7d4 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2070,14 +2070,12 @@ static int npf_interception(struct kvm_vcpu *vcpu)
> svm->vmcb->control.insn_len);
>
> /*
> - * rc == 0 indicates a userspace exit is needed to handle page
> - * transitions, so do that first before updating the RMP table.
> + * rc < 0 indicates a userspace exit may be needed to handle page
> + * attribute updates, so deal with that first before handling other
> + * potential RMP fault conditions.
> */
> - if (error_code & PFERR_GUEST_RMP_MASK) {
> - if (rc == 0)
> - return rc;
> + if (rc >= 0 && error_code & PFERR_GUEST_RMP_MASK)
This isn't correct either. A return of '0' also indiciates "exit to userspace",
it just doesn't happen with SNP because '0' is returned only when KVM attempts
emulation, and that too gets short-circuited by svm_check_emulate_instruction().
And I would honestly drop the comment, KVM's less-than-pleasant 1/0/-errno return
values overload is ubiquitous enough that it should be relatively self-explanatory.
Or if you prefer to keep a comment, drop the part that specifically calls out
attributes updates, because that incorrectly implies that's the _only_ reason
why KVM checks the return. But my vote is to drop the comment, because it
essentially becomes "don't proceed to step 2 if step 1 failed", which kind of
makes the reader go "well, yeah".
next prev parent reply other threads:[~2024-05-10 13:58 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-05-01 8:51 [PATCH v15 00/20] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support Michael Roth
2024-05-01 8:51 ` [PATCH v15 01/20] Revert "KVM: x86: Add gmem hook for determining max NPT mapping level" Michael Roth
2024-05-01 8:51 ` [PATCH v15 02/20] KVM: x86: Add hook for determining max NPT mapping level Michael Roth
2024-05-02 23:11 ` Isaku Yamahata
2024-05-07 17:48 ` Paolo Bonzini
2024-05-01 8:51 ` [PATCH v15 03/20] KVM: SEV: Select KVM_GENERIC_PRIVATE_MEM when CONFIG_KVM_AMD_SEV=y Michael Roth
2024-05-01 8:51 ` [PATCH v15 04/20] KVM: SEV: Add initial SEV-SNP support Michael Roth
2024-05-01 8:51 ` [PATCH v15 05/20] KVM: SEV: Add KVM_SEV_SNP_LAUNCH_START command Michael Roth
2024-05-01 8:51 ` [PATCH v15 06/20] KVM: SEV: Add KVM_SEV_SNP_LAUNCH_UPDATE command Michael Roth
2024-05-01 8:51 ` [PATCH v15 07/20] KVM: SEV: Add KVM_SEV_SNP_LAUNCH_FINISH command Michael Roth
2024-05-01 8:51 ` [PATCH v15 08/20] KVM: SEV: Add support to handle GHCB GPA register VMGEXIT Michael Roth
2024-05-01 8:51 ` [PATCH v15 09/20] KVM: SEV: Add support to handle MSR based Page State Change VMGEXIT Michael Roth
2024-05-16 8:28 ` Binbin Wu
2024-05-16 17:23 ` Paolo Bonzini
2024-05-21 0:49 ` Binbin Wu
2024-05-21 21:49 ` Michael Roth
2024-05-27 12:25 ` Binbin Wu
2024-05-28 10:39 ` Paolo Bonzini
2024-05-29 20:02 ` Sean Christopherson
2024-05-31 1:22 ` Binbin Wu
2024-05-31 13:10 ` Paolo Bonzini
2024-05-30 16:47 ` Zhi Wang
2024-05-01 8:52 ` [PATCH v15 10/20] KVM: SEV: Add support to handle " Michael Roth
2024-05-01 8:52 ` [PATCH v15 11/20] KVM: SEV: Add support to handle RMP nested page faults Michael Roth
2024-05-01 8:52 ` [PATCH v15 12/20] KVM: SEV: Support SEV-SNP AP Creation NAE event Michael Roth
2024-05-01 8:52 ` [PATCH v15 13/20] KVM: SEV: Implement gmem hook for initializing private pages Michael Roth
2024-05-20 10:16 ` Huang, Kai
2024-05-20 17:35 ` Sean Christopherson
2024-05-20 21:57 ` Huang, Kai
2024-05-20 23:15 ` Sean Christopherson
2024-05-20 23:41 ` Huang, Kai
2024-05-21 0:30 ` Sean Christopherson
2024-05-20 19:14 ` Isaku Yamahata
2024-05-01 8:52 ` [PATCH v15 14/20] KVM: SEV: Implement gmem hook for invalidating " Michael Roth
2024-05-01 8:52 ` [PATCH v15 15/20] KVM: x86: Implement hook for determining max NPT mapping level Michael Roth
2024-05-01 8:52 ` [PATCH v15 16/20] KVM: SEV: Avoid WBINVD for HVA-based MMU notifications for SNP Michael Roth
2024-05-01 8:52 ` [PATCH v15 17/20] KVM: SVM: Add module parameter to enable SEV-SNP Michael Roth
2024-05-01 8:52 ` [PATCH v15 18/20] KVM: SEV: Provide support for SNP_GUEST_REQUEST NAE event Michael Roth
2024-05-01 8:52 ` [PATCH v15 19/20] KVM: SEV: Provide support for SNP_EXTENDED_GUEST_REQUEST " Michael Roth
2024-05-13 23:48 ` Sean Christopherson
2024-05-14 2:51 ` Michael Roth
2024-05-14 14:36 ` Sean Christopherson
2024-05-15 1:25 ` [PATCH] KVM: SEV: Replace KVM_EXIT_VMGEXIT with KVM_EXIT_SNP_REQ_CERTS Michael Roth
2024-05-01 8:52 ` [PATCH v15 20/20] crypto: ccp: Add the SNP_VLEK_LOAD command Michael Roth
2024-05-07 18:04 ` [PATCH v15 00/20] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support Paolo Bonzini
2024-05-07 18:14 ` Michael Roth
2024-05-10 2:34 ` Michael Roth
2024-05-10 1:58 ` [PATCH v15 21/23] KVM: MMU: Disable fast path for private memslots Michael Roth
2024-05-10 1:58 ` [PATCH v15 22/23] KVM: SEV: Fix return code interpretation for RMP nested page faults Michael Roth
2024-05-10 13:58 ` Sean Christopherson [this message]
2024-05-10 15:36 ` Michael Roth
2024-05-10 16:01 ` Paolo Bonzini
2024-05-10 16:37 ` Michael Roth
2024-05-10 16:59 ` Paolo Bonzini
2024-05-10 17:25 ` Paolo Bonzini
2024-05-14 8:10 ` Borislav Petkov
2024-05-10 1:58 ` [PATCH v15 23/23] KVM: SEV: Fix PSC handling for SMASH/UNSMASH and partial update ops Michael Roth
2024-05-10 17:09 ` Paolo Bonzini
2024-05-10 19:08 ` Michael Roth
2024-05-10 13:47 ` [PATCH v15 21/23] KVM: MMU: Disable fast path for private memslots Sean Christopherson
2024-05-10 13:50 ` Paolo Bonzini
2024-05-10 15:27 ` Michael Roth
2024-05-10 15:59 ` Sean Christopherson
2024-05-10 17:47 ` Isaku Yamahata
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