linux-kernel - Re: [RFC PATCH v4 4/7] xen/pvh: Move Xen specific PVH VM initialization out of common code

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Date:   Thu, 1 Mar 2018 09:43:49 -0800
From:   Maran Wilson <maran.wilson@...cle.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>, pbonzini@...hat.com,
        jgross@...e.com, roger.pau@...rix.com, andrew.cooper3@...rix.com,
        hch@...radead.org, JBeulich@...e.com, x86@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        rkrcmar@...hat.com, jpoimboe@...hat.com, bp@...e.de,
        kirill.shutemov@...ux.intel.com, thomas.lendacky@....com,
        luto@...nel.org, dave.hansen@...ux.intel.com, davem@...emloft.net,
        gregkh@...uxfoundation.org, mchehab@...nel.org,
        linus.walleij@...aro.org, rdunlap@...radead.org
Subject: Re: [RFC PATCH v4 4/7] xen/pvh: Move Xen specific PVH VM
 initialization out of common code

On 3/1/2018 8:05 AM, Boris Ostrovsky wrote:
> On 02/28/2018 01:28 PM, Maran Wilson wrote:
>> We need to refactor PVH entry code so that support for other hypervisors
>> like Qemu/KVM can be added more easily.
>>
>> This patch moves the small block of code used for initializing Xen PVH
>> virtual machines into the Xen specific file. This initialization is not
>> going to be needed for Qemu/KVM guests. Moving it out of the common file
>> is going to allow us to compile kernels in the future without CONFIG_XEN
>> that are still capable of being booted as a Qemu/KVM guest via the PVH
>> entry point.
>>
>> Signed-off-by: Maran Wilson <maran.wilson@...cle.com>
>> ---
>>   arch/x86/pvh.c               | 28 ++++++++++++++++++++--------
>>   arch/x86/xen/enlighten_pvh.c | 18 +++++++++++++++++-
>>   2 files changed, 37 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/pvh.c b/arch/x86/pvh.c
>> index b56cb5e7d6ac..2d7a7f4958cb 100644
>> --- a/arch/x86/pvh.c
>> +++ b/arch/x86/pvh.c
>> @@ -72,26 +72,38 @@ static void __init init_pvh_bootparams(void)
>>   	pvh_bootparams.hdr.type_of_loader = (9 << 4) | 0; /* Xen loader */
>>   }
>>   
>> +/*
>> + * If we are trying to boot a Xen PVH guest, it is expected that the kernel
>> + * will have been configured to provide the required override for this routine.
>> + */
>> +void __init __weak xen_pvh_init(void)
>> +{
>> +	xen_raw_printk("Error: Missing xen PVH initialization\n");
> I think this should be printk (or, more precisely, this should not be
> xen_raw_printk()): we are here because we are *not* a Xen guest and so
> Xen-specific printk will not work. (and the same is true for the next
> patch where weak mem_map_via_hcall() is added).

Actually I left that xen_raw_printk() statement in on purpose. It's also 
possible that some future developer accidentally drops or hides the 
strong version of the routine even when CONFIG_XEN (and CONFIG_HVC_XEN) 
is enabled. In that situation, this error message might prove helpful in 
quickly identifying the problem when he or she attempts to boot a Xen guest.

And in situations where CONFIG_XEN is disabled or someone is booting a 
non xen guest, the statement simply becomes a nop, so no harm is done. 
And also, I believe this code is far too early for normal printk() 
statements to work so switching to that won't buy us anything.

Thanks,
-Maran

> -boris
>
>
>> +	BUG();
>> +}
>>