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From: Liran Alon <liran.alon@oracle.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "Singh, Brijesh" <brijesh.singh@amd.com>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"rkrcmar@redhat.com" <rkrcmar@redhat.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>
Subject: Re: [PATCH 1/2] KVM: SVM: Fix workaround for AMD Errata 1096
Date: Tue, 16 Jul 2019 19:56:31 +0300	[thread overview]
Message-ID: <60D01C4B-EC2E-453E-B5F6-BBE8FA94E31D@oracle.com> (raw)
In-Reply-To: <20190716164151.GC1987@linux.intel.com>



> On 16 Jul 2019, at 19:41, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
> 
> On Tue, Jul 16, 2019 at 07:20:42PM +0300, Liran Alon wrote:
>> How can a SMAP fault occur when CPL==3? One of the conditions for SMAP is
>> that CPL<3.
> 
> The CPU is effectively at CPL0 when it does the decode assist, e.g.:
> 
>  1. CPL3 guest hits reserved bit NPT fault (MMIO access)
>  2. CPU transitions to CPL0 on VM-Exit
>  3. CPU performs data access on **%rip**, encounters SMAP violation
>  4. CPU squashes SMAP violation, sets VMCB.insn_len=0
>  5. CPU delivers VM-Exit to software for original NPT fault
> 
> The original NPT fault is due to a reserved bit (or not present) entry for
> a MMIO GPA, *not* the GPA corresponding to %rip.  The fault on the decode
> assist is never delivered to software, it simply results in having invalid
> info in the VMCB's insn_bytes and insn_len fields.

If the CPU performs the VMExit transition of state before doing the data read for DecodeAssist,
then I agree that CPL will be 0 on data-access regardless of vCPU CPL. But this also means that SMAP
violation should be raised based on host CR4.SMAP value and not vCPU CR4.SMAP value as KVM code checks.

Furthermore, vCPU CPL of guest doesn’t need to be 3 in order to trigger this Errata.
It’s only important that guest page-tables maps the guest RIP as user-accessible. i.e. U/S bit in PTE set to 1.

So I’m still left a bit confused rather the correctness of KVM code handling this Errata.

-Liran



  reply	other threads:[~2019-07-16 16:57 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-15 20:30 KVM: SVM: Fix workaround for AMD Errata 1096 Liran Alon
2019-07-15 20:30 ` [PATCH 1/2] " Liran Alon
2019-07-16 15:48   ` Singh, Brijesh
2019-07-16 15:56     ` Liran Alon
2019-07-16 16:07       ` Liran Alon
2019-07-16 16:10       ` Singh, Brijesh
2019-07-16 16:20         ` Liran Alon
2019-07-16 16:41           ` Sean Christopherson
2019-07-16 16:56             ` Liran Alon [this message]
2019-07-16 17:27               ` Sean Christopherson
2019-07-16 17:27               ` Paolo Bonzini
2019-07-16 17:35                 ` Liran Alon
2019-07-16 19:28                   ` Singh, Brijesh
2019-07-16 19:34                     ` Liran Alon
2019-07-16 19:39                       ` Paolo Bonzini
2019-07-16 19:45                         ` Sean Christopherson
2019-07-16 19:50                           ` Liran Alon
2019-07-16 19:47                         ` Liran Alon
2019-07-16 19:41                       ` Sean Christopherson
2019-07-16 19:52                         ` Liran Alon
2019-07-16 20:02                       ` Singh, Brijesh
2019-07-16 20:07                         ` Sean Christopherson
2019-07-16 20:13                           ` Paolo Bonzini
2019-07-16 20:09                         ` Liran Alon
2019-07-16 20:27                           ` Singh, Brijesh
2019-07-16 20:54                             ` Sean Christopherson
2019-07-16 21:53                               ` Liran Alon
2019-07-16 18:05           ` Singh, Brijesh
2019-07-16 18:06             ` Singh, Brijesh
2019-07-15 20:30 ` [PATCH 2/2] KVM: x86: Rename need_emulation_on_page_fault() to handle_no_insn_on_page_fault() Liran Alon
2019-07-16 15:48   ` Sean Christopherson
2019-07-16 16:01     ` Liran Alon
2019-07-16 16:10       ` Sean Christopherson
2019-07-16 19:33         ` Singh, Brijesh

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