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t=1596246309; bh=sjM/fAEGxClBOS+U8ynUmUP8+Aqqz5vPbhJDNMh/upo=; h=References:In-Reply-To:From:Date:Subject:To:Cc:From; b=F1SkbLbUJ63XFPRTdKYmubCKNKuEDUn5TJzuwqzFh27nWgv3Rr/Uql7ap51TY6DiB qppf/kYJ0H7ph2yfZ8MdEFAaHqL48XD2XHiWnKGm7vQVhlDW671FZNWLt5H0Ofw0nx jW3YfBAep/edgRAqc1YIvwdTIjnwfAbjQ+zglJQA= Received: by mail-ed1-f44.google.com with SMTP id v22so12781973edy.0 for ; Fri, 31 Jul 2020 18:45:09 -0700 (PDT) X-Gm-Message-State: AOAM53042DQKJ/LR6SxntuvkOeT4FAxCxFqW7r3hTF25KlI5monqeX6D 2WO862untzsjwlKBfuUDaFX7eBIFDCw1XdcSL30l+Q== X-Google-Smtp-Source: ABdhPJyh2ARx8wi/UN9nL/otfHQZd5+RGHN7quC5P7hkzkPIb5B7BkSWz9rivA5myA2qOsQl6oEslOsrzUBxG9umpGs= X-Received: by 2002:adf:fa85:: with SMTP id h5mr5822401wrr.18.1596246306953; Fri, 31 Jul 2020 18:45:06 -0700 (PDT) MIME-Version: 1.0 References: <1594684087-61184-1-git-send-email-fenghua.yu@intel.com> <1594684087-61184-10-git-send-email-fenghua.yu@intel.com> In-Reply-To: <1594684087-61184-10-git-send-email-fenghua.yu@intel.com> From: Andy Lutomirski Date: Fri, 31 Jul 2020 18:44:55 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v6 09/12] x86/process: Clear PASID state for a newly forked/cloned thread To: Fenghua Yu Cc: Peter Zijlstra , Dave Hansen , H Peter Anvin , Jean-Philippe Brucker , Dave Jiang , Ashok Raj , x86 , amd-gfx , Christoph Hellwig , Ingo Molnar , Ravi V Shankar , Borislav Petkov , Thomas Gleixner , Tony Luck , Felix Kuehling , linux-kernel , iommu , Jacob Jun Pan , David Woodhouse X-BeenThere: iommu@lists.linux-foundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Development issues for Linux IOMMU support List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: iommu-bounces@lists.linux-foundation.org Sender: "iommu" On Mon, Jul 13, 2020 at 4:48 PM Fenghua Yu wrote: > > The PASID state has to be cleared on forks, since the child has a > different address space. The PASID is also cleared for thread clone. While > it would be correct to inherit the PASID in this case, it is unknown > whether the new task will use ENQCMD. Giving it the PASID "just in case" > would have the downside of increased context switch overhead to setting > the PASID MSR. > > Since #GP faults have to be handled on any threads that were created before > the PASID was assigned to the mm of the process, newly created threads > might as well be treated in a consistent way. Just how much context switch overhead are we talking about here? Unless the CPU works differently than I expect, I would guess you are saving exactly zero cycles. What am I missing? --Andy > > Suggested-by: Thomas Gleixner > Signed-off-by: Fenghua Yu > Reviewed-by: Tony Luck > --- > v2: > - Modify init_task_pasid(). > > arch/x86/kernel/process.c | 18 ++++++++++++++++++ > 1 file changed, 18 insertions(+) > > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c > index f362ce0d5ac0..1b1492e337a6 100644 > --- a/arch/x86/kernel/process.c > +++ b/arch/x86/kernel/process.c > @@ -121,6 +121,21 @@ static int set_new_tls(struct task_struct *p, unsigned long tls) > return do_set_thread_area_64(p, ARCH_SET_FS, tls); > } > > +/* Initialize the PASID state for the forked/cloned thread. */ > +static void init_task_pasid(struct task_struct *task) > +{ > + struct ia32_pasid_state *ppasid; > + > + /* > + * Initialize the PASID state so that the PASID MSR will be > + * initialized to its initial state (0) by XRSTORS when the task is > + * scheduled for the first time. > + */ > + ppasid = get_xsave_addr(&task->thread.fpu.state.xsave, XFEATURE_PASID); > + if (ppasid) > + ppasid->pasid = INIT_PASID; > +} > + > int copy_thread_tls(unsigned long clone_flags, unsigned long sp, > unsigned long arg, struct task_struct *p, unsigned long tls) > { > @@ -174,6 +189,9 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp, > task_user_gs(p) = get_user_gs(current_pt_regs()); > #endif > > + if (static_cpu_has(X86_FEATURE_ENQCMD)) > + init_task_pasid(p); > + > /* Set a new TLS for the child thread? */ > if (clone_flags & CLONE_SETTLS) > ret = set_new_tls(p, tls); > -- > 2.19.1 > _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu