> -----Original Message----- > From: kvm-owner@xxxxxxxxxxxxxxx <kvm-owner@xxxxxxxxxxxxxxx> On Behalf > Of Jim Mattson > Sent: Friday, November 1, 2019 3:08 PM > To: Moger, Babu <Babu.Moger@xxxxxxx> > Cc: Andy Lutomirski <luto@xxxxxxxxxx>; tglx@xxxxxxxxxxxxx; > mingo@xxxxxxxxxx; bp@xxxxxxxxx; hpa@xxxxxxxxx; pbonzini@xxxxxxxxxx; > rkrcmar@xxxxxxxxxx; sean.j.christopherson@xxxxxxxxx; vkuznets@xxxxxxxxxx; > wanpengli@xxxxxxxxxxx; x86@xxxxxxxxxx; joro@xxxxxxxxxx; > zohar@xxxxxxxxxxxxx; yamada.masahiro@xxxxxxxxxxxxx; > nayna@xxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx > Subject: Re: [PATCH 2/4] kvm: svm: Enable UMIP feature on AMD > > On Fri, Nov 1, 2019 at 1:04 PM Moger, Babu <Babu.Moger@xxxxxxx> wrote: > > > > > > > > On 11/1/19 2:24 PM, Andy Lutomirski wrote: > > > On Fri, Nov 1, 2019 at 12:20 PM Moger, Babu <Babu.Moger@xxxxxxx> > wrote: > > >> > > >> > > >> > > >> On 11/1/19 1:29 PM, Jim Mattson wrote: > > >>> On Fri, Nov 1, 2019 at 10:33 AM Moger, Babu <Babu.Moger@xxxxxxx> > wrote: > > >>>> > > >>>> AMD 2nd generation EPYC processors support UMIP (User-Mode > Instruction > > >>>> Prevention) feature. The UMIP feature prevents the execution of certain > > >>>> instructions if the Current Privilege Level (CPL) is greater than 0. > > >>>> If any of these instructions are executed with CPL > 0 and UMIP > > >>>> is enabled, then kernel reports a #GP exception. > > >>>> > > >>>> The idea is taken from articles: > > >>>> https://lwn.net/Articles/738209/ > > >>>> https://lwn.net/Articles/694385/ > > >>>> > > >>>> Enable the feature if supported on bare metal and emulate instructions > > >>>> to return dummy values for certain cases. > > >>>> > > >>>> Signed-off-by: Babu Moger <babu.moger@xxxxxxx> > > >>>> --- > > >>>> arch/x86/kvm/svm.c | 21 ++++++++++++++++----- > > >>>> 1 file changed, 16 insertions(+), 5 deletions(-) > > >>>> > > >>>> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c > > >>>> index 4153ca8cddb7..79abbdeca148 100644 > > >>>> --- a/arch/x86/kvm/svm.c > > >>>> +++ b/arch/x86/kvm/svm.c > > >>>> @@ -2533,6 +2533,11 @@ static void > svm_decache_cr4_guest_bits(struct kvm_vcpu *vcpu) > > >>>> { > > >>>> } > > >>>> > > >>>> +static bool svm_umip_emulated(void) > > >>>> +{ > > >>>> + return boot_cpu_has(X86_FEATURE_UMIP); > > >>>> +} > > >>> > > >>> This makes no sense to me. If the hardware actually supports UMIP, > > >>> then it doesn't have to be emulated. > > >> My understanding.. > > >> > > >> If the hardware supports the UMIP, it will generate the #GP fault when > > >> these instructions are executed at CPL > 0. Purpose of the emulation is to > > >> trap the GP and return a dummy value. Seems like this required in certain > > >> legacy OSes running in protected and virtual-8086 modes. In long mode no > > >> need to emulate. Here is the bit explanation > https://lwn.net/Articles/738209/ > > >> > > > > > > Indeed. Again, what does this have to do with your patch? > > > > > >> > > >>> > > >>> To the extent that kvm emulates UMIP on Intel CPUs without hardware > > >>> UMIP (i.e. smsw is still allowed at CPL>0), we can always do the same > > >>> emulation on AMD, because SVM has always offered intercepts of sgdt, > > >>> sidt, sldt, and str. So, if you really want to offer this emulation on > > >>> pre-EPYC 2 CPUs, this function should just return true. But, I have to > > >>> ask, "why?" > > >> > > >> > > >> Trying to support UMIP feature only on EPYC 2 hardware. No intention to > > >> support pre-EPYC 2. > > >> > > > > > > I think you need to totally rewrite your changelog to explain what you > > > are doing. > > > > > > As I understand it, there are a couple of things KVM can do: > > > > > > 1. If the underlying hardware supports UMIP, KVM can expose UMIP to > > > the guest. SEV should be irrelevant here. > > > > > > 2. Regardless of whether the underlying hardware supports UMIP, KVM > > > can try to emulate UMIP in the guest. This may be impossible if SEV > > > is enabled. > > > > > > Which of these are you doing? > > > > > My intention was to do 1. Let me go back and think about this again. > > (1) already works. That’s right. Thanks Jim and Andy. How about updating the Kconfig (patch #4) and update it to CONFIG_X86_UMIP (instead of CONFIG_X86_INTEL_UMIP). Right now, it appears it is supported on only Intel.
