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

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Date:   Mon, 12 Mar 2018 17:03:36 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
        x86@...nel.org
Cc:     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

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);
                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);
	...

?

Paolo