Re: [PATCH RFC 1/4] xen PVonHVM: use E820_Reserved area for shared_info

[Konrad Rzeszutek Wilk]

On Tue, Jul 15, 2014 at 03:40:37PM +0200, Vitaly Kuznetsov wrote:
> From: Olaf Hering <olaf@xxxxxxxxx>
> 
> This is a respin of 00e37bdb0113a98408de42db85be002f21dbffd3
> ("xen PVonHVM: move shared_info to MMIO before kexec").
> 
> Currently kexec in a PVonHVM guest fails with a triple fault because the
> new kernel overwrites the shared info page. The exact failure depends on
> the size of the kernel image. This patch moves the pfn from RAM into an
> E820 reserved memory area.
> 
> The pfn containing the shared_info is located somewhere in RAM. This will
> cause trouble if the current kernel is doing a kexec boot into a new
> kernel. The new kernel (and its startup code) can not know where the pfn
> is, so it can not reserve the page. The hypervisor will continue to update
> the pfn, and as a result memory corruption occours in the new kernel.
> 
> The toolstack marks the memory area FC000000-FFFFFFFF as reserved in the
> E820 map. Within that range newer toolstacks (4.3+) will keep 1MB
> starting from FE700000 as reserved for guest use. Older Xen4 toolstacks
> will usually not allocate areas up to FE700000, so FE700000 is expected
> to work also with older toolstacks.
> 
> In Xen3 there is no reserved area at a fixed location. If the guest is
> started on such old hosts the shared_info page will be placed in RAM. As
> a result kexec can not be used.

So this looks right, the one thing that we really need to check 
is e9daff24a266307943457086533041bd971d0ef9

   This reverts commit 9d02b43dee0d7fb18dfb13a00915550b1a3daa9f.
    
    We are doing this b/c on 32-bit PVonHVM with older hypervisors
    (Xen 4.1) it ends up bothing up the start_info. This is bad b/c
    we use it for the time keeping, and the timekeeping code loops
    forever - as the version field never changes. Olaf says to
    revert it, so lets do that.
    

Could you kindly test that the migration on 32-bit PVHVM guests
on older hypervisors works?

> 
> Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> (cherry picked from commit 9d02b43dee0d7fb18dfb13a00915550b1a3daa9f)
> 
> [On resume we need to reset the xen_vcpu_info, which the original
> patch did not do]
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  arch/x86/xen/enlighten.c | 74 ++++++++++++++++++++++++++++++++++++------------
>  arch/x86/xen/suspend.c   |  2 +-
>  arch/x86/xen/xen-ops.h   |  2 +-
>  3 files changed, 58 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index ffb101e..a11af62 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -1726,23 +1726,29 @@ asmlinkage __visible void __init xen_start_kernel(void)
>  #endif
>  }
>  
> -void __ref xen_hvm_init_shared_info(void)
> +#ifdef CONFIG_XEN_PVHVM
> +#define HVM_SHARED_INFO_ADDR 0xFE700000UL
> +static struct shared_info *xen_hvm_shared_info;
> +static unsigned long xen_hvm_sip_phys;
> +static int xen_major, xen_minor;
> +
> +static void xen_hvm_connect_shared_info(unsigned long pfn)
>  {
> -	int cpu;
>  	struct xen_add_to_physmap xatp;
> -	static struct shared_info *shared_info_page = 0;
>  
> -	if (!shared_info_page)
> -		shared_info_page = (struct shared_info *)
> -			extend_brk(PAGE_SIZE, PAGE_SIZE);
>  	xatp.domid = DOMID_SELF;
>  	xatp.idx = 0;
>  	xatp.space = XENMAPSPACE_shared_info;
> -	xatp.gpfn = __pa(shared_info_page) >> PAGE_SHIFT;
> +	xatp.gpfn = pfn;
>  	if (HYPERVISOR_memory_op(XENMEM_add_to_physmap, &xatp))
>  		BUG();
>  
> -	HYPERVISOR_shared_info = (struct shared_info *)shared_info_page;
> +}
> +static void __init xen_hvm_set_shared_info(struct shared_info *sip)
> +{
> +	int cpu;
> +
> +	HYPERVISOR_shared_info = sip;
>  
>  	/* xen_vcpu is a pointer to the vcpu_info struct in the shared_info
>  	 * page, we use it in the event channel upcall and in some pvclock
> @@ -1760,20 +1766,39 @@ void __ref xen_hvm_init_shared_info(void)
>  	}
>  }
>  
> -#ifdef CONFIG_XEN_PVHVM
> +/* Reconnect the shared_info pfn to a (new) mfn */
> +void xen_hvm_resume_shared_info(void)
> +{
> +	xen_hvm_connect_shared_info(xen_hvm_sip_phys >> PAGE_SHIFT);
> +	xen_hvm_set_shared_info(xen_hvm_shared_info);
> +}
> +
> +/* Xen tools prior to Xen 4 do not provide a E820_Reserved area for guest usage.
> + * On these old tools the shared info page will be placed in E820_Ram.
> + * Xen 4 provides a E820_Reserved area at 0xFC000000, and this code expects
> + * that nothing is mapped up to HVM_SHARED_INFO_ADDR.
> + * Xen 4.3+ provides an explicit 1MB area at HVM_SHARED_INFO_ADDR which is used
> + * here for the shared info page. */
> +static void __init xen_hvm_init_shared_info(void)
> +{
> +	if (xen_major < 4) {
> +		xen_hvm_shared_info = extend_brk(PAGE_SIZE, PAGE_SIZE);
> +		xen_hvm_sip_phys = __pa(xen_hvm_shared_info);
> +	} else {
> +		xen_hvm_sip_phys = HVM_SHARED_INFO_ADDR;
> +		set_fixmap(FIX_PARAVIRT_BOOTMAP, xen_hvm_sip_phys);
> +		xen_hvm_shared_info =
> +		(struct shared_info *)fix_to_virt(FIX_PARAVIRT_BOOTMAP);
> +	}
> +	xen_hvm_resume_shared_info();
> +}
> +
>  static void __init init_hvm_pv_info(void)
>  {
> -	int major, minor;
> -	uint32_t eax, ebx, ecx, edx, pages, msr, base;
> +	uint32_t  ecx, edx, pages, msr, base;
>  	u64 pfn;
>  
>  	base = xen_cpuid_base();
> -	cpuid(base + 1, &eax, &ebx, &ecx, &edx);
> -
> -	major = eax >> 16;
> -	minor = eax & 0xffff;
> -	printk(KERN_INFO "Xen version %d.%d.\n", major, minor);
> -
>  	cpuid(base + 2, &pages, &msr, &ecx, &edx);
>  
>  	pfn = __pa(hypercall_page);
> @@ -1828,10 +1853,23 @@ static void __init xen_hvm_guest_init(void)
>  
>  static uint32_t __init xen_hvm_platform(void)
>  {
> +	uint32_t eax, ebx, ecx, edx, base;
> +
>  	if (xen_pv_domain())
>  		return 0;
>  
> -	return xen_cpuid_base();
> +	base = xen_cpuid_base();
> +	if (!base)
> +		return 0;
> +
> +	cpuid(base + 1, &eax, &ebx, &ecx, &edx);
> +
> +	xen_major = eax >> 16;
> +	xen_minor = eax & 0xffff;
> +
> +	printk(KERN_INFO "Xen version %d.%d.\n", xen_major, xen_minor);
> +
> +	return 1;
>  }
>  
>  bool xen_hvm_need_lapic(void)
> diff --git a/arch/x86/xen/suspend.c b/arch/x86/xen/suspend.c
> index c4df9db..8d6793e 100644
> --- a/arch/x86/xen/suspend.c
> +++ b/arch/x86/xen/suspend.c
> @@ -32,7 +32,7 @@ static void xen_hvm_post_suspend(int suspend_cancelled)
>  {
>  #ifdef CONFIG_XEN_PVHVM
>  	int cpu;
> -	xen_hvm_init_shared_info();
> +	xen_hvm_resume_shared_info();
>  	xen_callback_vector();
>  	xen_unplug_emulated_devices();
>  	if (xen_feature(XENFEAT_hvm_safe_pvclock)) {
> diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
> index 97d8765..d083e82 100644
> --- a/arch/x86/xen/xen-ops.h
> +++ b/arch/x86/xen/xen-ops.h
> @@ -43,7 +43,7 @@ void xen_enable_syscall(void);
>  void xen_vcpu_restore(void);
>  
>  void xen_callback_vector(void);
> -void xen_hvm_init_shared_info(void);
> +void xen_hvm_resume_shared_info(void);
>  void xen_unplug_emulated_devices(void);
>  
>  void __init xen_build_dynamic_phys_to_machine(void);
> -- 
> 1.9.3
> 
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