linux-kernel - Re: [PATCH RFC v7 07/64] KVM: SEV: Handle KVM_HC_MAP_GPA

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Message-ID: <20230131141506.GA14449@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date:   Tue, 31 Jan 2023 06:15:07 -0800
From:   Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
To:     Michael Roth <michael.roth@....com>
Cc:     kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
        linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        vkuznets@...hat.com, wanpengli@...cent.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,
        dovmurik@...ux.ibm.com, tobin@....com, bp@...en8.de,
        vbabka@...e.cz, kirill@...temov.name, ak@...ux.intel.com,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
        dgilbert@...hat.com, jarkko@...nel.org, ashish.kalra@....com,
        harald@...fian.com, Nikunj A Dadhania <nikunj@....com>
Subject: Re: [PATCH RFC v7 07/64] KVM: SEV: Handle KVM_HC_MAP_GPA_RANGE
 hypercall

On Fri, Jan 27, 2023 at 08:35:58AM -0800, Jeremi Piotrowski wrote:
> On Wed, Dec 14, 2022 at 01:39:59PM -0600, Michael Roth wrote:
> > From: Nikunj A Dadhania <nikunj@....com>
> > 
> > KVM_HC_MAP_GPA_RANGE hypercall is used by the SEV guest to notify a
> > change in the page encryption status to the hypervisor.
> > 
> > The hypercall exits to userspace with KVM_EXIT_HYPERCALL exit code,
> > currently this is used for explicit memory conversion between
> > shared/private for memfd based private memory.
> > 
> > Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> > Signed-off-by: Michael Roth <michael.roth@....com>
> > ---
> >  arch/x86/kvm/x86.c  | 8 ++++++++
> >  virt/kvm/kvm_main.c | 1 +
> >  2 files changed, 9 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index bb6adb216054..732f9cbbadb5 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -9649,6 +9649,7 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> 
> Couldn't find a better commit to comment on:
> when the guest has the ptp-kvm module, it will issue a KVM_HC_CLOCK_PAIRING
> hypercall. This will pass sev_es_validate_vmgexit validation and end up in this
> function where kvm_pv_clock_pairing() is called, and that calls
> kvm_write_guest(). This results in a CPU soft-lockup, at least in my testing.
> 
> Are there any emulated hypercalls that make sense for snp guests? We should
> block at least the ones that definitely don't work.
> 
> Jeremi

So turns out the soft-lockup is a nested issue (details here for those
interested: [^1]), but the questions still stands, of whether we should
block kvm_write_page (and similar) explicitly or rely on the rmp fault.

[^1]: https://github.com/jepio/linux/commit/6c3bdf552e93664ae172660e24ceceed60fd4df5