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SubjectRe: [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.

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