linux-kernel - Re: [PATCH v3 1/2] KVM: X86: Processor States following Reset or INIT

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Date:   Wed, 11 Oct 2017 20:11:28 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v3 1/2] KVM: X86: Processor States following Reset or INIT

2017-10-11 19:36 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2017-10-11 08:39+0800, Wanpeng Li:
>> 2017-10-11 2:55 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>> > 2017-10-09 15:51-0700, Wanpeng Li:
>> >> From: Wanpeng Li <wanpeng.li@...mail.com>
>> >>
>> >> - XCR0 is reset to 1 by RESET but not INIT
>> >> - XSS is zeroed by both RESET and INIT
>> >> - BNDCFGU, BND0-BND3, BNDCFGS are zeroed by both RESET and INIT
>> >>
>> >> This patch does this according to SDM.
>> >>
>> >> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> >> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> >> Cc: Jim Mattson <jmattson@...gle.com>
>> >> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> >> ---
>> >> v2 -> v3:
>> >>  * fix null pointer deference
>> >>  * fix patch description
>> >> v1 -> v2:
>> >>  * XCR0 is not zeroed by INIT
>> >>  * XSS, BNDCFGU, BND0-BND3, BNDCFGS are zeroed by both RESET and INIT
>> >>
>> >>  arch/x86/kvm/vmx.c |  2 ++
>> >>  arch/x86/kvm/x86.c | 15 +++++++++++++++
>> >>  2 files changed, 17 insertions(+)
>> >>
>> >> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> >> @@ -7804,18 +7804,33 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>> >>       kvm_async_pf_hash_reset(vcpu);
>> >>       vcpu->arch.apf.halted = false;
>> >>
>> >> +     if (kvm_mpx_supported()) {
>> >> +             void *mpx_state_buffer;
>> >> +
>> >> +             mpx_state_buffer = get_xsave_addr(&vcpu->arch.guest_fpu.state.xsave, XFEATURE_MASK_BNDREGS);
>> >> +             if (mpx_state_buffer)
>> >> +                     memset(mpx_state_buffer, 0, sizeof(struct mpx_bndreg_state));
>> >
>> > I think we should call kvm_put_guest_fpu() before doing this.
>> > The register might be loaded in CPU and XSAVE instruction from
>> > vcpu_put() would overwrite any changes we did.
>>
>> Except in vCPU creation path, vcpu_put() is called in the
>> destroy/error path, so I think it doesn't matter to overwrite changes
>> we did.
>
> Yeah, creation doesn't have FPU loaded and destroy doesn't matter in any
> case, but we also have the INIT path from kvm_apic_accept_events() that
> can be called with loaded FPU and does not let userspace fix the state.

Thanks for pointing out this. :)

>
>> >> +             mpx_state_buffer = get_xsave_addr(&vcpu->arch.guest_fpu.state.xsave, XFEATURE_MASK_BNDCSR);
>> >> +             if (mpx_state_buffer)
>> >> +                     memset(mpx_state_buffer, 0, sizeof(u64));
>> >
>> > XFEATURE_MASK_BNDCSR is actually
>> >
>> >   struct mpx_bndcsr {
>> >         u64 bndcfgu;
>> >         u64 bndstatus;
>> >   } __packed;
>> >
>> > So clearing two u64 would be correct, thanks.
>>
>> SDM section 9.1.1 just mentioned BNDCFGU will be cleared after
>> RESET/INIT, but not mentioned BNDSTATUS.
>
> That section conspires ;) I found a mention in 17.3.3 Configuration and
> Status Registers:
>
>   RESET or INIT# will set BNDCFGx and BNDSTATUS registers to zero.

Agreed.

Regards,
Wanpeng Li