linux-kernel - RE: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush

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Message-ID: <CY4PR21MB0631989FA0C9135AAD2DD1F8F1890@CY4PR21MB0631.namprd21.prod.outlook.com>
Date:   Fri, 11 Aug 2017 01:15:18 +0000
From:   Jork Loeser <Jork.Loeser@...rosoft.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     KY Srinivasan <kys@...rosoft.com>,
        Simon Xiao <sixiao@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "luto@...nel.org" <luto@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>
Subject: RE: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB
 flush

> -----Original Message-----
> From: Peter Zijlstra [mailto:peterz@...radead.org]
> Sent: Thursday, August 10, 2017 12:28
> To: Jork Loeser <Jork.Loeser@...rosoft.com>
> Cc: KY Srinivasan <kys@...rosoft.com>; Simon Xiao <sixiao@...rosoft.com>;
> Haiyang Zhang <haiyangz@...rosoft.com>; Stephen Hemminger
> <sthemmin@...rosoft.com>; torvalds@...ux-foundation.org; luto@...nel.org;
> hpa@...or.com; vkuznets@...hat.com; linux-kernel@...r.kernel.org;
> rostedt@...dmis.org; andy.shevchenko@...il.com; tglx@...utronix.de;
> mingo@...nel.org; linux-tip-commits@...r.kernel.org
> Subject: Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush

> > > > Hold on.. if we don't IPI for TLB invalidation. What serializes
> > > > our software page table walkers like fast_gup() ?
> > >
> > > Hypervisor may implement this functionality via an IPI.
> > >
> > > K. Y
> >
> > HvFlushVirtualAddressList() states:
> > This call guarantees that by the time control returns back to the
> > caller, the observable effects of all flushes on the specified virtual
> > processors have occurred.
> >
> > HvFlushVirtualAddressListEx() refers to HvFlushVirtualAddressList() as adding
> sparse target VP lists.
> >
> > Is this enough of a guarantee, or do you see other races?
> 
> That's nowhere near enough. We need the remote CPU to have completed any
> guest IF section that was in progress at the time of the call.
> 
> So if a host IPI can interrupt a guest while the guest has IF cleared, and we then
> process the host IPI -- clear the TLBs -- before resuming the guest, which still has
> IF cleared, we've got a problem.
> 
> Because at that point, our software page-table walker, that relies on IF being
> clear to guarantee the page-tables exist, because it holds off the TLB invalidate
> and thereby the freeing of the pages, gets its pages ripped out from under it.

I see, IF is used as a locking mechanism for the pages. Would CONFIG_HAVE_RCU_TABLE_FREE be an option for x86? There are caveats (statically enabled, RCU for page-free), yet if the resulting perf is still a gain it would be worthwhile for Hyper-V targeted kernels.

Regards,
Jork