linux-kernel - Re: [PATCH 1/1] x86/xen: Reset VCPU0 info pointer after shared

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Message-ID: <B3C02FE4-651D-4A89-90BB-78E389764688@amazon.com>
Date:   Mon, 7 May 2018 18:00:09 +0000
From:   "van der Linden, Frank" <fllinden@...zon.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "jgross@...e.com" <jgross@...e.com>
Subject: Re: [PATCH 1/1] x86/xen: Reset VCPU0 info pointer after shared_info
 remap

Hi Boris,

Thanks for the feedback.

On 5/7/18, 8:13 AM, "Boris Ostrovsky" <boris.ostrovsky@...cle.com> wrote:

    > diff --git a/arch/x86/xen/enlighten_hvm.c b/arch/x86/xen/enlighten_hvm.c
    > index 6b424da1ce75..c78b3e8fb2e5 100644
    > --- a/arch/x86/xen/enlighten_hvm.c
    > +++ b/arch/x86/xen/enlighten_hvm.c
    > @@ -71,6 +71,19 @@ static void __init xen_hvm_init_mem_mapping(void)
    >  {
    >  	early_memunmap(HYPERVISOR_shared_info, PAGE_SIZE);
    >  	HYPERVISOR_shared_info = __va(PFN_PHYS(shared_info_pfn));
    > +
    > +	/*
    > +	 * The virtual address of the shared_info page has changed, so
    > +	 * the vcpu_info pointer for VCPU 0 is now stale.
    
    Is it "has changed" or "has changed if kaslr is on"?

It's "has changed".  See commit 4ca83dcf4e3bc0c98836dbb97553792ca7ea5429 .  It's a way to make kaslr work, but it's done regardless of whether it's enabled or not.
 
    > +	 *
    > +	 * The prepare_boot_cpu callback will re-initialize it via
    > +	 * xen_vcpu_setup, but we can't rely on that to be called for
    > +	 * old Xen versions (xen_have_vector_callback == 0).
    > +	 *
    > +	 * It is, in any case, bad to have a stale vcpu_info pointer
    > +	 * so reset it now.
    > +	 */
    > +	xen_vcpu_info_reset(0);
    
    
    Why not xen_vcpu_setup(0)?
    
Basically, I wanted to be minimally invasive. xen_vcpu_setup does a little more work (tries to do the VCPU placement hypercall), and will be called later in any case. So doing just the basic xen_vcpu_info_reset for VCPU 0 seems like the best way to do it; it just re-iterates what is done for VCPU 0 earlier in boot, which is also a vcpu_info_reset.

Frank