From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752739AbeCMNUT (ORCPT ); Tue, 13 Mar 2018 09:20:19 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:59364 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752268AbeCMNUG (ORCPT ); Tue, 13 Mar 2018 09:20:06 -0400 From: Vitaly Kuznetsov To: Paolo Bonzini Cc: kvm@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, Radim =?utf-8?B?S3LEjW3DocWZ?= , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andy Lutomirski Subject: Re: [PATCH 1/3] x86/kvm/vmx: read MSR_FS_BASE from current->thread References: <20180312140300.6166-1-vkuznets@redhat.com> <20180312140300.6166-2-vkuznets@redhat.com> <327f3194-715c-7aa7-f1bb-9ee1073f5b99@redhat.com> Date: Tue, 13 Mar 2018 14:20:03 +0100 In-Reply-To: <327f3194-715c-7aa7-f1bb-9ee1073f5b99@redhat.com> (Paolo Bonzini's message of "Mon, 12 Mar 2018 17:03:36 +0100") Message-ID: <87ina0cdto.fsf@vitty.brq.redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.3 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Paolo Bonzini 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