linux-kernel - Re: [PATCH 1/3] x86/kvm/vmx: read MSR_FS

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Date:   Mon, 12 Mar 2018 17:18:13 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        kvm list <kvm@...r.kernel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 1/3] x86/kvm/vmx: read MSR_FS_BASE from current->thread

On 12/03/2018 17:13, Andy Lutomirski wrote:
>>
>>         savesegment(fs, vmx->host_state.fs_sel);
>>         /*
>>          * When FSGSBASE extensions are enabled, this will have to use
>>          * RD{FS,GS}BASE instead of accessing current, and the
>>          * corresponding WR{FS,GS}BASE should be done unconditionally,
>>          * even if fs_reload_needed (resp. gs_ldt_reload_needed) is 1.
>>          */
>>         if (vmx->host_state.fs_sel <= 3) {
>>                 vmcs_write16(HOST_FS_SELECTOR, vmx->host_state.fs_sel);
>>                 vmcs_write16(HOST_FS_BASE, current->thread.fsbase);
>>                 vmx->host_state.fs_reload_needed = 0;
>>         } else {
>>                 vmcs_write16(HOST_FS_SELECTOR, 0);
>>                 vmcs_write16(HOST_FS_BASE, 0);
>>                 vmx->host_state.fs_reload_needed = 1;
>>         }
>>         savesegment(gs, vmx->host_state.gs_sel);
>>         ...
>>
>> ?
>>
> Hmm, probably, although this still gets the case where the user writes
> 0 to %fs wrong.  Of course, save_fsgs() also gets that wrong.
> 
> I'm okay with this variant as long as you add a comment to
> save_..._legacy pointing at the KVM code.

Why in save_..._legacy?  If it is about FSGSBASE, shouldn't it be in
save_fsgs?  (Or if not I'm missing what the comment should be about).

Paolo