linux-kernel - Re: [RFC 02/16] x86/kvm: Introduce KVM memory protection feature

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Date:   Wed, 27 May 2020 10:39:33 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Will Drewry <wad@...omium.org>,
        "Edgecombe\, Rick P" <rick.p.edgecombe@...el.com>,
        "Kleen\, Andi" <andi.kleen@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [RFC 02/16] x86/kvm: Introduce KVM memory protection feature

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> On Mon, May 25, 2020 at 06:15:25PM +0300, Kirill A. Shutemov wrote:
>> On Mon, May 25, 2020 at 04:58:51PM +0200, Vitaly Kuznetsov wrote:
>> > > @@ -727,6 +734,15 @@ static void __init kvm_init_platform(void)
>> > >  {
>> > >  	kvmclock_init();
>> > >  	x86_platform.apic_post_init = kvm_apic_init;
>> > > +
>> > > +	if (kvm_para_has_feature(KVM_FEATURE_MEM_PROTECTED)) {
>> > > +		if (kvm_hypercall0(KVM_HC_ENABLE_MEM_PROTECTED)) {
>> > > +			pr_err("Failed to enable KVM memory protection\n");
>> > > +			return;
>> > > +		}
>> > > +
>> > > +		mem_protected = true;
>> > > +	}
>> > >  }
>> > 
>> > Personally, I'd prefer to do this via setting a bit in a KVM-specific
>> > MSR instead. The benefit is that the guest doesn't need to remember if
>> > it enabled the feature or not, it can always read the config msr. May
>> > come handy for e.g. kexec/kdump.
>> 
>> I think we would need to remember it anyway. Accessing MSR is somewhat
>> expensive. But, okay, I can rework it MSR if needed.
>
> I think Vitaly is talking about the case where the kernel can't easily get
> at its cached state, e.g. after booting into a new kernel.  The kernel would
> still have an X86_FEATURE bit or whatever, providing a virtual MSR would be
> purely for rare slow paths.
>
> That being said, a hypercall plus CPUID bit might be better, e.g. that'd
> allow the guest to query the state without risking a #GP.

We have rdmsr_safe() for that! :-) MSR (and hypercall to that matter)
should have an associated CPUID feature bit of course.

Yes, hypercall + CPUID would do but normally we treat CPUID data as
static and in this case we'll make it a dynamically flipping
bit. Especially if we introduce 'KVM_HC_DISABLE_MEM_PROTECTED' later.

>
>> Note, that we can avoid the enabling algother, if we modify BIOS to deal
>> with private/shared memory. Currently BIOS get system crash if we enable
>> the feature from time zero.
>
> Which would mesh better with a CPUID feature bit.
>

And maybe even help us to resolve 'reboot' problem.

-- 
Vitaly