linux-kernel - Re: xen_exit

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Date:   Thu, 27 Apr 2017 09:21:40 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        Juergen Gross <jgross@...e.com>, X86 ML <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: xen_exit_mmap() questions


>>>>
>>>>> Also, this code in drop_other_mm_ref() looks dubious to me:
>>>>>
>>>>>     /* If this cpu still has a stale cr3 reference, then make sure
>>>>>        it has been flushed. */
>>>>>     if (this_cpu_read(xen_current_cr3) == __pa(mm->pgd))
>>>>>         load_cr3(swapper_pg_dir);
>>>>>
>>>>> If cr3 hasn't been flushed to the hypervisor because we're in a lazy
>>>>> mode, why would load_cr3() help?  Shouldn't this be xen_mc_flush()
>>>>> instead?
>>>>
>>>> load_cr3() actually ends with xen_mc_flush() by way of xen_write_cr3()
>>>> -> xen_mc_issue().
>>>
>>> xen_mc_issue() does:
>>>
>>>         if ((paravirt_get_lazy_mode() & mode) == 0)
>>>                 xen_mc_flush();
>>>
>>> I assume the load_cr3() is intended to deal with the case where we're
>>> in lazy mode, but we'll still be in lazy mode, right?  Or does it
>>> serve some other purpose?
>>
>> Of course. I can't read (I ignored the "== 0" part).
>>
>> Apparently the early version had an explicit flush but then it disappeared
>> (commit 9f79991d4186089e228274196413572cc000143b).
>>
>> The point of CR3 loading here, I believe, is to make sure the hypervisor
>> knows that the (v)CPU is no longer using the the mm's cr3 (we are loading
>> swapper_pgdir here).
> But that's what leave_mm() does.  To be fair, the x86 lazy TLB
> management is a big mess, and this came up because I'm trying to clean
> it up without removing it.

True. I don't know though if you can guarantee that leave_mm() (or
load_cr3() inside it) is actually called if we are in lazy mode.

>
> I suppose I can try to keep xen_exit_mmap() working.  Is there a
> simple way to try to unpin but to not treat it as an error if the
> hypervisor rejects it?

Even if we managed to craft a call in Linux to do this (current
xen_pgd_unpin() will result in a WARNing in xen_mc_flush()) this will
still cause a bunch of warnings in the hypervisor (if it is built as
DEBUG, but bad nevertheless).

But even without that, it is an error for a reason so how are you
planning to continue if you ignore it?

-boris