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

[Vitaly Kuznetsov]

Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> writes:

> 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?
>

Sure, will do! Was there anything special about the setup or any 32-bit
pvhvm guest migration (on 64-bit hypervisor I suppose) would fail? I can
try checking both current and old versions to make sure the issue was
acutually fixed.

>> 
>> 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
>> 

-- 
  Vitaly
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