Subject: Re: [PATCH 18/26] KVM: PPC: KVM PV guest stubs

From: Matt Evans <[email-masked]>
Date: Mon, 28 Jun 2010 14:39:48 +1000

Howdy Alex!

Alexander Graf wrote:
> We will soon start and replace instructions from the text section with
> other, paravirtualized versions. To ease the readability of those patches
> I split out the generic looping and magic page mapping code out.
> 
> This patch still only contains stubs. But at least it loops through the
> text section :).
> 
> Signed-off-by: Alexander Graf <[email-masked]>
> ---
>  arch/powerpc/kernel/kvm.c |   59 +++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 59 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kvm.c b/arch/powerpc/kernel/kvm.c
> index 2d8dd73..d873bc6 100644
> --- a/arch/powerpc/kernel/kvm.c
> +++ b/arch/powerpc/kernel/kvm.c
> @@ -32,3 +32,62 @@
>  #define KVM_MAGIC_PAGE		(-4096L)
>  #define magic_var(x) KVM_MAGIC_PAGE + offsetof(struct kvm_vcpu_arch_shared, x)
>  
> +static bool kvm_patching_worked = true;
> +
> +static void kvm_map_magic_page(void *data)
> +{
> +	kvm_hypercall2(KVM_HC_PPC_MAP_MAGIC_PAGE,
> +		       KVM_MAGIC_PAGE,  /* Physical Address */
> +		       KVM_MAGIC_PAGE); /* Effective Address */
> +}
> +
> +static void kvm_check_ins(u32 *inst)
> +{
> +	u32 _inst = *inst;
> +	u32 inst_no_rt = _inst & ~KVM_MASK_RT;
> +	u32 inst_rt = _inst & KVM_MASK_RT;
> +
> +	switch (inst_no_rt) {
> +	}
> +
> +	switch (_inst) {
> +	}
> +
> +	flush_icache_range((ulong)inst, (ulong)inst + 4);
> +}
> +
> +static void kvm_use_magic_page(void)
> +{
> +	u32 *p;
> +	u32 *start, *end;
> +
> +	/* Tell the host to map the magic page to -4096 on all CPUs */
> +
> +	on_each_cpu(kvm_map_magic_page, NULL, 1);
> +
> +	/* Now loop through all code and find instructions */
> +
> +	start = (void*)_stext;
> +	end = (void*)_etext;
> +
> +	for (p = start; p < end; p++)
> +		kvm_check_ins(p);
> +}

Could you do something similar in module_finalize() to patch loaded modules' .text sections?

> +
> +static int __init kvm_guest_init(void)
> +{
> +	char *p;
> +
> +	if (!kvm_para_available())
> +		return 0;
> +
> +	if (kvm_para_has_feature(KVM_FEATURE_MAGIC_PAGE))
> +		kvm_use_magic_page();
> +
> +	printk(KERN_INFO "KVM: Live patching for a fast VM %s\n",
> +			 kvm_patching_worked ? "worked" : "failed");
> +
> +	return 0;
> +}
> +
> +postcore_initcall(kvm_guest_init);

Cheers,

Matt