linux-kernel - Re: [RFC 2/7] KVM: VMX: Expose IA32

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Message-ID: <268b0ee4-e56f-981c-c03e-6dca8a4e99da@intel.com>
Date:   Tue, 18 Aug 2020 15:27:51 +0800
From:   Chenyi Qiang <chenyi.qiang@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC 2/7] KVM: VMX: Expose IA32_PKRS MSR



On 8/14/2020 1:31 AM, Jim Mattson wrote:
> On Wed, Aug 12, 2020 at 10:42 PM Chenyi Qiang <chenyi.qiang@...el.com> wrote:
>>
>>
>>
>> On 8/13/2020 5:21 AM, Jim Mattson wrote:
>>> On Fri, Aug 7, 2020 at 1:46 AM Chenyi Qiang <chenyi.qiang@...el.com> wrote:
>>>>
>>>> Protection Keys for Supervisor Pages (PKS) uses IA32_PKRS MSR (PKRS) at
>>>> index 0x6E1 to allow software to manage supervisor protection key
>>>> rights. For performance consideration, PKRS intercept will be disabled
>>>> so that the guest can access the PKRS without VM exits.
>>>> PKS introduces dedicated control fields in VMCS to switch PKRS, which
>>>> only does the retore part. In addition, every VM exit saves PKRS into
>>>> the guest-state area in VMCS, while VM enter won't save the host value
>>>> due to the expectation that the host won't change the MSR often. Update
>>>> the host's value in VMCS manually if the MSR has been changed by the
>>>> kernel since the last time the VMCS was run.
>>>> The function get_current_pkrs() in arch/x86/mm/pkeys.c exports the
>>>> per-cpu variable pkrs_cache to avoid frequent rdmsr of PKRS.
>>>>
>>>> Signed-off-by: Chenyi Qiang <chenyi.qiang@...el.com>
>>>> ---
>>>
>>>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>>>> index 11e4df560018..df2c2e733549 100644
>>>> --- a/arch/x86/kvm/vmx/nested.c
>>>> +++ b/arch/x86/kvm/vmx/nested.c
>>>> @@ -289,6 +289,7 @@ static void vmx_sync_vmcs_host_state(struct vcpu_vmx *vmx,
>>>>           dest->ds_sel = src->ds_sel;
>>>>           dest->es_sel = src->es_sel;
>>>>    #endif
>>>> +       dest->pkrs = src->pkrs;
>>>
>>> Why isn't this (and other PKRS code) inside the #ifdef CONFIG_X86_64?
>>> PKRS isn't usable outside of long mode, is it?
>>>
>>
>> Yes, I'm also thinking about whether to put all pks code into
>> CONFIG_X86_64. The kernel implementation also wrap its pks code inside
>> CONFIG_ARCH_HAS_SUPERVISOR_PKEYS which has dependency with CONFIG_X86_64.
>> However, maybe this can help when host kernel disable PKS but the guest
>> enable it. What do you think about this?
> 
> I see no problem in exposing PKRS to the guest even if the host
> doesn't have CONFIG_ARCH_HAS_SUPERVISOR_PKEYS.
> 

Yes, but I would prefer to keep it outside CONFIG_X86_64. PKS code has 
several code blocks and putting them under x86_64 may end up being a 
mess. In addition, PKU KVM related code isn't under CONFIG_X86_64 as 
well. So, is it really necessary to put inside?