linux-kernel - Re: [patch 3/3] x86/paravirt: Replace paravirt patch asm magic

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Date:   Thu, 25 Apr 2019 10:19:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [patch 3/3] x86/paravirt: Replace paravirt patch asm magic

On Thu, Apr 25, 2019 at 10:08:10AM +0200, Peter Zijlstra wrote:
> On Thu, Apr 25, 2019 at 08:52:09AM +0200, Ingo Molnar wrote:
> 
> > > -# ifdef CONFIG_PARAVIRT_XXL
> > > -DEF_NATIVE(irq, irq_disable, "cli");
> > > -DEF_NATIVE(irq, irq_enable, "sti");
> > > -DEF_NATIVE(irq, restore_fl, "push %eax; popf");
> > > -DEF_NATIVE(irq, save_fl, "pushf; pop %eax");
> > > -DEF_NATIVE(cpu, iret, "iret");
> > > -DEF_NATIVE(mmu, read_cr2, "mov %cr2, %eax");
> > > -DEF_NATIVE(mmu, write_cr3, "mov %eax, %cr3");
> > > -DEF_NATIVE(mmu, read_cr3, "mov %cr3, %eax");
> > 
> > > +static const struct patch_xxl patch_data_xxl = {
> > > +	.irq_irq_disable	= { 0xfa },		// cli
> > > +	.irq_irq_enable		= { 0xfb },		// sti
> > > +	.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
> > > +	.irq_restore_fl		= { 0x57, 0x9d },	// push %rdi; popfq
> > > +	.mmu_write_cr3		= { 0x0f, 0x22, 0xdf },	// mov %rdi, %cr3
> > > +	.cpu_wbinvd		= { 0x0f, 0x09 },	// wbinvd
> > > +	.cpu_usergs_sysret64	= { 0x0f, 0x01, 0xf8,
> > > +				    0x48, 0x0f, 0x07 },	// swapgs; sysretq
> > > +	.cpu_swapgs		= { 0x0f, 0x01, 0xf8 },	// swapgs
> > > +	.mov64			= { 0x48, 0x89, 0xf8 },	// mov %rdi, %rax
> > > +# else
> > > +	.irq_restore_fl		= { 0x50, 0x9d },	// push %eax; popf
> > > +	.mmu_write_cr3		= { 0x0f, 0x22, 0xd8 },	// mov %eax, %cr3
> > > +	.cpu_iret		= { 0xcf },		// iret
> > > +# endif
> > 
> > I think these open-coded hexa versions are somewhat fragile as well, how 
> > about putting these into a .S file and controlling the sections in an LTO 
> > safe manner there?
> > 
> > That will also allow us to write proper asm, and global labels can be 
> > used to extract the patchlets and their length?
> 
> While I'm not fan either; I think that will be worse still, because it
> splits the information over multiple files.
> 
> The advantage of this form is that it is clear how long the instructions
> are, which is important for the patching. These immediates have to be
> shorter than 5 bytes because they overwrite the CALL/JMP to the paravirt
> function.
> 
> /me eyes .cpu_usergs_sysret64 and goes wtf..

OK, my bad, the indirect CALL/JMP is 6 bytes.. phew.