Re: Speeding up VMX with GDT fixmap trickery?

[Paolo Bonzini]

On 09/06/2017 03:13, Andy Lutomirski wrote:
> Hi all-
> 
> As promised when Thomas did his GDT fixmap work, here is a draft patch
> to speed up KVM by extending it.
> 
> The downside of this patch is that it makes the fixmap significantly
> larger on 64-bit systems if NR_CPUS is large (it adds 15 more pages
> per CPU).  I don't know if we care at all.  It also bloats the kernel
> image by 4k and wastes 4k of RAM for the entire time the system is
> booted.  We could avoid the latter bit (sort of) by not mapping the
> extra fixmap pages at all and handling the resulting faults somehow.
> That would scare me -- now we have IRET generating #PF when running
> malicious , and that way lies utter madness.
> 
> The upside is that we don't need to do LGDT after a vmexit on VMX.
> LGDT is slooooooooooow.  But no, I haven't benchmarked this yet.
> 
> What do you all think?
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/kvm&id=e249a09787d6956b52d8260b2326d8f12f768799

Not sure I understand this completely, but:

 /* Get the fixmap index for a specific processor */
 static inline unsigned int get_cpu_gdt_ro_index(int cpu)
 {
-	return FIX_GDT_REMAP_BEGIN + cpu;
+	return FIX_GDT_REMAP_END - cpu * PAGES_PER_GDT;
 }

isn't this off by one.  I think it should be

	FIX_GDT_REMAP_END + 1 - cpu * PAGES_PER_GDT

or just FIX_GDT_REMAP_BEGIN + cpu * PAGES_PER_GDT?  That is for example:

	FIX_GDT_REMAP_BEGIN = 100
	get_cpu_gdt_ro_index(0) = 100
	get_cpu_gdt_ro_index(1) = 116
	get_cpu_gdt_ro_index(2) = 132
	get_cpu_gdt_ro_index(3) = 148
	FIX_GDT_REMAP_END = 163

Paolo