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Date: Mon, 19 Sep 2022 23:46:55 +0000
From: Ashish Kalra <ashkalra@....com>
To: Tom Lendacky <thomas.lendacky@....com>,
Alper Gun <alpergun@...gle.com>
Cc: Peter Gonda <pgonda@...gle.com>,
Ashish Kalra <Ashish.Kalra@....com>,
the arch/x86 maintainers <x86@...nel.org>,
LKML <linux-kernel@...r.kernel.org>,
kvm list <kvm@...r.kernel.org>, linux-coco@...ts.linux.dev,
Linux Memory Management List <linux-mm@...ck.org>,
Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
"H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <seanjc@...gle.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Jim Mattson <jmattson@...gle.com>,
Andy Lutomirski <luto@...nel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Sergio Lopez <slp@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
David Rientjes <rientjes@...gle.com>,
Dov Murik <dovmurik@...ux.ibm.com>,
Tobin Feldman-Fitzthum <tobin@....com>,
Borislav Petkov <bp@...en8.de>,
Michael Roth <michael.roth@....com>,
Vlastimil Babka <vbabka@...e.cz>,
"Kirill A . Shutemov" <kirill@...temov.name>,
Andi Kleen <ak@...ux.intel.com>,
Tony Luck <tony.luck@...el.com>, Marc Orr <marcorr@...gle.com>,
Sathyanarayanan Kuppuswamy
<sathyanarayanan.kuppuswamy@...ux.intel.com>,
"Dr. David Alan Gilbert" <dgilbert@...hat.com>, jarkko@...nel.org
Subject: Re: [PATCH Part2 v6 37/49] KVM: SVM: Add support to handle MSR based
Page State Change VMGEXIT
On 9/19/22 22:18, Tom Lendacky wrote:
> On 9/19/22 17:02, Alper Gun wrote:
>> On Mon, Sep 19, 2022 at 2:38 PM Tom Lendacky
>> <thomas.lendacky@....com> wrote:
>>>
>>> On 9/19/22 12:53, Alper Gun wrote:
>>>> On Fri, Aug 19, 2022 at 9:54 AM Peter Gonda <pgonda@...gle.com> wrote:
>>>>>
>>>>>> +
>>>>>> +static int __snp_handle_page_state_change(struct kvm_vcpu *vcpu,
>>>>>> enum psc_op op, gpa_t gpa,
>>>>>> + int level)
>>>>>> +{
>>>>>> + struct kvm_sev_info *sev = &to_kvm_svm(vcpu->kvm)->sev_info;
>>>>>> + struct kvm *kvm = vcpu->kvm;
>>>>>> + int rc, npt_level;
>>>>>> + kvm_pfn_t pfn;
>>>>>> + gpa_t gpa_end;
>>>>>> +
>>>>>> + gpa_end = gpa + page_level_size(level);
>>>>>> +
>>>>>> + while (gpa < gpa_end) {
>>>>>> + /*
>>>>>> + * If the gpa is not present in the NPT then
>>>>>> build the NPT.
>>>>>> + */
>>>>>> + rc = snp_check_and_build_npt(vcpu, gpa, level);
>>>>>> + if (rc)
>>>>>> + return -EINVAL;
>>>>>> +
>>>>>> + if (op == SNP_PAGE_STATE_PRIVATE) {
>>>>>> + hva_t hva;
>>>>>> +
>>>>>> + if (snp_gpa_to_hva(kvm, gpa, &hva))
>>>>>> + return -EINVAL;
>>>>>> +
>>>>>> + /*
>>>>>> + * Verify that the hva range is
>>>>>> registered. This enforcement is
>>>>>> + * required to avoid the cases where a
>>>>>> page is marked private
>>>>>> + * in the RMP table but never gets
>>>>>> cleanup during the VM
>>>>>> + * termination path.
>>>>>> + */
>>>>>> + mutex_lock(&kvm->lock);
>>>>>> + rc = is_hva_registered(kvm, hva,
>>>>>> page_level_size(level));
>>>>>> + mutex_unlock(&kvm->lock);
>>>>>> + if (!rc)
>>>>>> + return -EINVAL;
>>>>>> +
>>>>>> + /*
>>>>>> + * Mark the userspace range unmerable
>>>>>> before adding the pages
>>>>>> + * in the RMP table.
>>>>>> + */
>>>>>> + mmap_write_lock(kvm->mm);
>>>>>> + rc = snp_mark_unmergable(kvm, hva,
>>>>>> page_level_size(level));
>>>>>> + mmap_write_unlock(kvm->mm);
>>>>>> + if (rc)
>>>>>> + return -EINVAL;
>>>>>> + }
>>>>>> +
>>>>>> + write_lock(&kvm->mmu_lock);
>>>>>> +
>>>>>> + rc = kvm_mmu_get_tdp_walk(vcpu, gpa, &pfn,
>>>>>> &npt_level);
>>>>>> + if (!rc) {
>>>>>> + /*
>>>>>> + * This may happen if another vCPU
>>>>>> unmapped the page
>>>>>> + * before we acquire the lock. Retry the
>>>>>> PSC.
>>>>>> + */
>>>>>> + write_unlock(&kvm->mmu_lock);
>>>>>> + return 0;
>>>>>> + }
>>>>>
>>>>> I think we want to return -EAGAIN or similar if we want the caller to
>>>>> retry, right? I think returning 0 here hides the error.
>>>>>
>>>>
>>>> The problem here is that the caller(linux guest kernel) doesn't retry
>>>> if PSC fails. The current implementation in the guest kernel is that
>>>> if a page state change request fails, it terminates the VM with
>>>> GHCB_TERM_PSC reason.
>>>> Returning 0 here is not a good option because it will fail the PSC
>>>> silently and will probably cause a nested RMP fault later. Returning
>>>
>>> Returning 0 here is ok because the PSC current index into the PSC
>>> structure will not be updated and the guest will then retry (see the
>>> loop
>>> in vmgexit_psc() in arch/x86/kernel/sev.c).
>>>
>>> Thanks,
>>> Tom
>>
>> But the host code updates the index. It doesn't leave the loop because
>> rc is 0. The guest will think that it is successful.
>> rc = __snp_handle_page_state_change(vcpu, op, gpa, level);
>> if (rc)
>> goto out;
>>
>> Also the page state change request with MSR is not retried. It
>> terminates the VM if the MSR request fails.
>
> Ah, right. I see what you mean. It should probably return a -EAGAIN
> instead of 0 and then the if (rc) check should be modified to
> specifically look for -EAGAIN and goto out after setting rc to 0.
>
> But that does leave the MSR protocol open to the problem that you
> mention, so, yes, retry logic in snp_handle_page_state_change() for a
> -EAGAIN seems reasonable.
>
> Thanks,
> Tom
I believe it makes more sense to add the retry logic within
__snp_handle_page_state_change() itself, as that will make it work for
both the GHCB MSR protocol and the GHCB VMGEXIT requests.
Thanks, Ashish
>
>>
>>>
>>>> an error also terminates the guest immediately with current guest
>>>> implementation. I think the best approach here is adding a retry logic
>>>> to this function. Retrying without returning an error should help it
>>>> work because snp_check_and_build_npt will be called again and in the
>>>> second attempt this should work.
>>>>
>>>>>> +
>>>>>> + /*
>>>>>> + * Adjust the level so that we don't go higher
>>>>>> than the backing
>>>>>> + * page level.
>>>>>> + */
>>>>>> + level = min_t(size_t, level, npt_level);
>>>>>> +
>>>>>> + trace_kvm_snp_psc(vcpu->vcpu_id, pfn, gpa, op,
>>>>>> level);
>>>>>> +
>>>>>> + switch (op) {
>>>>>> + case SNP_PAGE_STATE_SHARED:
>>>>>> + rc = snp_make_page_shared(kvm, gpa, pfn,
>>>>>> level);
>>>>>> + break;
>>>>>> + case SNP_PAGE_STATE_PRIVATE:
>>>>>> + rc = rmp_make_private(pfn, gpa, level,
>>>>>> sev->asid, false);
>>>>>> + break;
>>>>>> + default:
>>>>>> + rc = -EINVAL;
>>>>>> + break;
>>>>>> + }
>>>>>> +
>>>>>> + write_unlock(&kvm->mmu_lock);
>>>>>> +
>>>>>> + if (rc) {
>>>>>> + pr_err_ratelimited("Error op %d gpa %llx
>>>>>> pfn %llx level %d rc %d\n",
>>>>>> + op, gpa, pfn, level, rc);
>>>>>> + return rc;
>>>>>> + }
>>>>>> +
>>>>>> + gpa = gpa + page_level_size(level);
>>>>>> + }
>>>>>> +
>>>>>> + return 0;
>>>>>> +}
>>>>>> +
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