linux-kernel - Re: [PATCH v2 1/4] x86/xen: remove 32-bit Xen PV guest support

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Date:   Fri, 3 Jul 2020 07:15:27 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org
Cc:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH v2 1/4] x86/xen: remove 32-bit Xen PV guest support

On 03.07.20 00:59, Boris Ostrovsky wrote:
> On 7/1/20 7:06 AM, Juergen Gross wrote:
>> Xen is requiring 64-bit machines today and since Xen 4.14 it can be
>> built without 32-bit PV guest support. There is no need to carry the
>> burden of 32-bit PV guest support in the kernel any longer, as new
>> guests can be either HVM or PVH, or they can use a 64 bit kernel.
>>
>> Remove the 32-bit Xen PV support from the kernel.
>>
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> ---
>>   arch/x86/entry/entry_32.S      | 109 +----------
>>   arch/x86/include/asm/proto.h   |   2 +-
>>   arch/x86/include/asm/segment.h |   2 +-
>>   arch/x86/kernel/head_32.S      |  31 ---
>>   arch/x86/xen/Kconfig           |   3 +-
>>   arch/x86/xen/Makefile          |   3 +-
>>   arch/x86/xen/apic.c            |  17 --
>>   arch/x86/xen/enlighten_pv.c    |  48 +----
> 
> 
> Should we drop PageHighMem() test in set_aliased_prot()?
> 
> 
> (And there are few other places where is is used, in mmu_pv.c)

Yes, will drop those.

> 
> 
> 
>> @@ -555,13 +547,8 @@ static void xen_load_tls(struct thread_struct *t, unsigned int cpu)
>>   	 * exception between the new %fs descriptor being loaded and
>>   	 * %fs being effectively cleared at __switch_to().
>>   	 */
>> -	if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_CPU) {
>> -#ifdef CONFIG_X86_32
>> -		lazy_load_gs(0);
>> -#else
> 
> 
> I think this also needs an adjustment to the preceding comment.

Yes.

> 
> 
>>   
>> -#ifdef CONFIG_X86_PAE
>> -static void xen_set_pte_atomic(pte_t *ptep, pte_t pte)
>> -{
>> -	trace_xen_mmu_set_pte_atomic(ptep, pte);
>> -	__xen_set_pte(ptep, pte);
> 
> 
> Probably not for this series but I wonder whether __xen_set_pte() should
> continue to use hypercall now that we are 64-bit only.

As Andrew wrote already the hypercall will be cheaper.

I'll adjust the comment, though.

> 
> 
>> @@ -654,14 +621,12 @@ static int __xen_pgd_walk(struct mm_struct *mm, pgd_t *pgd,
> 
> 
> Comment above should be updated.

Yes.


Juergen