Re: [Xen-devel] incorrect layout of globals from head_64.S during kexec boot

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Sun, Jul 15, 2012 at 06:06:53PM +0200, Olaf Hering wrote:
> On Tue, Jul 10, Keir Fraser wrote:
> 
> > Best thing to do, is possible, is map the shared-info page in the
> > xen-platform pci device's BAR memory range. Then it will not conflict with
> > any RAM.
> 
> This patch does that. I did a kexec boot and a save/restore.
> It does not deal with the possible race were the old pfn is not backed
> by a mfn.
> 
> Olaf
> 
> 
> commit a9d5567c67a2317c9db174a4deef6c5690220749
> Author: Olaf Hering <olaf@aepfle.de>
> Date:   Thu Jul 12 19:38:39 2012 +0200
> 
>     xen PVonHVM: move shared info page from RAM to MMIO
>     
>     Signed-off-by: Olaf Hering <ohering@suse.de>
> 
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index ff962d4..8a743af 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -1465,23 +1465,22 @@ static int init_hvm_pv_info(int *major, int *minor)
>  	return 0;
>  }
>  
> -void __ref xen_hvm_init_shared_info(void)
> +static struct shared_info *hvm_shared_info;
> +static unsigned long hvm_shared_info_pfn;
> +

Please include a big comment explainig what this machination is OK
to do multiple times. If you can include the juicy snippets of the
email converstation in this comment so that in 6 months if somebody
looks at this they have a good understanding of it.

> +static void __ref set_shared_info(struct shared_info *sip, 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;
> +	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
> @@ -1494,8 +1493,48 @@ void __ref xen_hvm_init_shared_info(void)
>  	for_each_online_cpu(cpu) {
>  		per_cpu(xen_vcpu, cpu) = &HYPERVISOR_shared_info->vcpu_info[cpu];
>  	}
> +	mb();
>  }
>  
> +/* Reconnect the pfn to a mfn */
> +void __ref xen_hvm_resume_shared_info(void)
> +{
> +	set_shared_info(hvm_shared_info, hvm_shared_info_pfn);
> +}
> +
> +/* Move the pfn in RAM to a pfn in MMIO space */
> +void __ref xen_hvm_finalize_shared_info(struct shared_info *sip, unsigned long pfn)
> +{
> +	struct xen_memory_reservation reservation = {
> +		.domid = DOMID_SELF,
> +		.nr_extents = 1,
> +	};
> +	int rc;
> +
> +	set_xen_guest_handle(reservation.extent_start, &hvm_shared_info_pfn);
> +
> +	/* Move pfn, disconnects previous pfn from mfn */
> +	set_shared_info(sip, pfn);
> +
> +	/* Allocate new mfn for previous pfn */
> +	rc = HYPERVISOR_memory_op(XENMEM_populate_physmap, &reservation);
> +	WARN_ON(rc != 1);

Shouldn't you do some recovery if it fails? Like reassign the old RAM pfn
back to the shared page?

> +
> +	/* Remember final pfn and pointer for resume */
> +	hvm_shared_info_pfn = pfn;
> +	hvm_shared_info = sip;
> +}
> +
> +/* Use a pfn in RAM until PCI init is done. */
> +static void __ref xen_hvm_initial_shared_info(void)
> +{
> +	/* FIXME simply allocate a page and release it after pfn move (How at this stage?) */

You could just have an page on the .bss that has __init_data. During
bootup it would be recycled - and since the platform PCI driver is always
built in, it would not be a problem I think?

Thought if somebody does not compile with CONFIG_XEN_PVHVM then
it would make sense to keep the existing allocation from the .brk.

> +	hvm_shared_info = extend_brk(PAGE_SIZE, PAGE_SIZE);
> +	hvm_shared_info_pfn = __pa(hvm_shared_info) >> PAGE_SHIFT;
> +	set_shared_info(hvm_shared_info, hvm_shared_info_pfn);
> +}
> +
> +
>  #ifdef CONFIG_XEN_PVHVM
>  static int __cpuinit xen_hvm_cpu_notify(struct notifier_block *self,
>  				    unsigned long action, void *hcpu)
> @@ -1526,7 +1565,7 @@ static void __init xen_hvm_guest_init(void)
>  	if (r < 0)
>  		return;
>  
> -	xen_hvm_init_shared_info();

Put here a comment saying:
	/* Initaliazing a RAM PFN that is later going to be replaced with
	 * a MMIO PFN. */
> +	xen_hvm_initial_shared_info();
>  
>  	if (xen_feature(XENFEAT_hvm_callback_vector))
>  		xen_have_vector_callback = 1;
> diff --git a/arch/x86/xen/suspend.c b/arch/x86/xen/suspend.c
> index 45329c8..ae8a00c 100644
> --- a/arch/x86/xen/suspend.c
> +++ b/arch/x86/xen/suspend.c
> @@ -30,7 +30,7 @@ void xen_arch_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 202d4c1..1e4329e 100644
> --- a/arch/x86/xen/xen-ops.h
> +++ b/arch/x86/xen/xen-ops.h
> @@ -41,7 +41,7 @@ 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);
> diff --git a/drivers/xen/platform-pci.c b/drivers/xen/platform-pci.c
> index 97ca359..cbe866b 100644
> --- a/drivers/xen/platform-pci.c
> +++ b/drivers/xen/platform-pci.c
> @@ -108,6 +108,8 @@ static int __devinit platform_pci_init(struct pci_dev *pdev,
>  	long ioaddr;
>  	long mmio_addr, mmio_len;
>  	unsigned int max_nr_gframes;
> +	unsigned long addr;
> +	struct shared_info *hvm_shared_info;
>  
>  	if (!xen_domain())
>  		return -ENODEV;
> @@ -138,6 +140,11 @@ static int __devinit platform_pci_init(struct pci_dev *pdev,
>  	platform_mmio = mmio_addr;
>  	platform_mmiolen = mmio_len;
>  
> +	addr = alloc_xen_mmio(PAGE_SIZE);
> +	hvm_shared_info = ioremap(addr, PAGE_SIZE);
> +	memset(hvm_shared_info, 0, PAGE_SIZE);
> +	xen_hvm_finalize_shared_info(hvm_shared_info, addr >> PAGE_SHIFT);
> +
>  	if (!xen_have_vector_callback) {
>  		ret = xen_allocate_irq(pdev);
>  		if (ret) {
> diff --git a/include/xen/events.h b/include/xen/events.h
> index 04399b2..3337698 100644
> --- a/include/xen/events.h
> +++ b/include/xen/events.h
> @@ -58,6 +58,8 @@ void notify_remote_via_irq(int irq);
>  
>  void xen_irq_resume(void);
>  
> +void xen_hvm_finalize_shared_info(struct shared_info *sip, unsigned long pfn);
> +
>  /* Clear an irq's pending state, in preparation for polling on it */
>  void xen_clear_irq_pending(int irq);
>  void xen_set_irq_pending(int irq);
> 
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