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

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Date:   Tue, 13 Mar 2018 14:20:03 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, 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>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 1/3] x86/kvm/vmx: read MSR_FS_BASE from current->thread

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 12/03/2018 15:02, Vitaly Kuznetsov wrote:
>>  
>> +/*
>> + * Currently, the only way for processes to change their FS/GS base is to call
>> + * ARCH_SET_FS/GS prctls and these reflect changes they make in task->thread.
>> + * There are, however, additional considerations:
>> + *
>> + * There is X86_BUG_NULL_SEG: on some CPUs writing '0' to FS/GS selectors zeroes
>> + * the base and on some it doesn't, we need to check for that
>> + * (see save_base_legacy()).
>> + *
>> + * When FSGSBASE extensions are enabled userspace processes will be able to
>> + * change their FS/GS bases without kernel intervention. save_fsgs() will
>> + * have to be updated to actually read FS and GS bases with RD[FG,GS]BASE
>> + * instructions.
>> + */
>> +void save_current_fsgs(void)
>> +{
>> +	save_fsgs(current);
>> +}
>> +EXPORT_SYMBOL_GPL(save_current_fsgs);
>
> We don't really need save_fsgs in KVM though.  We already do the
> savesegment ourselves, and we know Intel CPUs don't have
> X86_BUG_NULL_SEG.  So this:
>
>         savesegment(fs, vmx->host_state.fs_sel);
>         if (!(vmx->host_state.fs_sel & 7)) {
>                 vmcs_write16(HOST_FS_SELECTOR, vmx->host_state.fs_sel);
>                 vmx->host_state.fs_reload_needed = 0;
>         } else {
>                 vmcs_write16(HOST_FS_SELECTOR, 0);
>                 vmx->host_state.fs_reload_needed = 1;
>         }
>         savesegment(gs, vmx->host_state.gs_sel);
> 	...
>
> could probably become simply:
>
>         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);

vmcs_writel() I guess ... and, just to make sure I follow your
suggestion, this is for x86_64 only, right? x86_32 does

vmcs_writel(HOST_FS_BASE, segment_base(vmx->host_state.fs_sel));

and I think it needs to stay.

(personally, I'm rather for exporting save_fsgs(), dropping
savesegment() and getting all we need from current to avoid propagating
assumptions but I'm flexible)

-- 
  Vitaly