linux-kernel - Re: [PATCH v3 4/7] x86/paravirt: remove 32-bit support from PARAVIRT

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Message-ID: <3a0712ee-efca-5684-d321-3b789e01fc16@suse.com>
Date:   Fri, 7 Aug 2020 12:02:59 +0200
From:   Jürgen Groß <jgross@...e.com>
To:     peterz@...radead.org
Cc:     xen-devel@...ts.xenproject.org, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        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>, Deep Shah <sdeep@...are.com>,
        "VMware, Inc." <pv-drivers@...are.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH v3 4/7] x86/paravirt: remove 32-bit support from
 PARAVIRT_XXL

On 07.08.20 11:39, peterz@...radead.org wrote:
> On Fri, Aug 07, 2020 at 10:38:23AM +0200, Juergen Gross wrote:
> 
>> -# else
>> -	const unsigned char	cpu_iret[1];
>> -# endif
>>   };
>>   
>>   static const struct patch_xxl patch_data_xxl = {
>> @@ -42,7 +38,6 @@ static const struct patch_xxl patch_data_xxl = {
>>   	.irq_save_fl		= { 0x9c, 0x58 },	// pushf; pop %[re]ax
>>   	.mmu_read_cr2		= { 0x0f, 0x20, 0xd0 },	// mov %cr2, %[re]ax
>>   	.mmu_read_cr3		= { 0x0f, 0x20, 0xd8 },	// mov %cr3, %[re]ax
>> -# ifdef CONFIG_X86_64
>>   	.mmu_write_cr3		= { 0x0f, 0x22, 0xdf },	// mov %rdi, %cr3
>>   	.irq_restore_fl		= { 0x57, 0x9d },	// push %rdi; popfq
>>   	.cpu_wbinvd		= { 0x0f, 0x09 },	// wbinvd
>> @@ -50,19 +45,11 @@ static const struct patch_xxl patch_data_xxl = {
>>   				    0x48, 0x0f, 0x07 },	// swapgs; sysretq
>>   	.cpu_swapgs		= { 0x0f, 0x01, 0xf8 },	// swapgs
>>   	.mov64			= { 0x48, 0x89, 0xf8 },	// mov %rdi, %rax
>> -# else
>> -	.mmu_write_cr3		= { 0x0f, 0x22, 0xd8 },	// mov %eax, %cr3
>> -	.irq_restore_fl		= { 0x50, 0x9d },	// push %eax; popf
>> -	.cpu_iret		= { 0xcf },		// iret
>> -# endif
> 
> I was looking at x86_64 paravirt the other day and found we actually
> have pv_ops.cpu.iret users there..

On x86_64 we have (without PARAVIRT_XXL):

#define INTERRUPT_RETURN        jmp native_iret

and with PARAVIRT_XXL this is basically a jmp *pv_ops.cpu.iret which
will then be patched to either jmp native_iret or jmp xen_iret.

On x86_32 INTERRUPT_RETURN was just "iret" for the non-paravirt case.
This is the reason for above dropping of the static patch data.

> So we want to change the above to also patch iret on x86_64 or do we
> need to fix x86_64 to not have pv-iret?

We want it to stay how it is. This will let both variants (PARVIRT y/n)
continue to work.


Juergen