linux-kernel - RE: [PATCH 1/3] PCI: hv: use the correct buffer size in new_pcichild

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Date:   Thu, 10 Nov 2016 17:05:08 +0000
From:   Jake Oshins <jakeo@...rosoft.com>
To:     Dexuan Cui <decui@...rosoft.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "devel@...uxdriverproject.org" <devel@...uxdriverproject.org>
CC:     "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        "Stephen Hemminger" <sthemmin@...rosoft.com>,
        Hadden Hoppert <haddenh@...rosoft.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "apw@...onical.com" <apw@...onical.com>,
        "olaf@...fle.de" <olaf@...fle.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/3] PCI: hv: use the correct buffer size in
 new_pcichild_device()

> -----Original Message-----
> 
> > From: Jake Oshins
> > > From: Dexuan Cui
> > > Sent: Wednesday, November 9, 2016 11:18 PM
> > > We don't really need such a big on-stack buffer.
> > > vmbus_sendpacket() here only uses sizeof(struct pci_child_message).
> > >
> > > @@ -1271,9 +1271,9 @@ static struct hv_pci_dev
> > > *new_pcichild_device(struct hv_pcibus_device *hbus,
> > >  	struct hv_pci_dev *hpdev;
> > >  	struct pci_child_message *res_req;
> > >  	struct q_res_req_compl comp_pkt;
> > > -	union {
> > > -	struct pci_packet init_packet;
> > > -		u8 buffer[0x100];
> > > +	struct {
> > > +		struct pci_packet init_packet;
> > > +		u8 buffer[sizeof(struct pci_child_message)];
> > >  	} pkt;
> > >  	unsigned long flags;
> > >  	int ret;
> >
> > This change seems good to me, in that it's always a bad idea to use too
> much
> > stack.  But this won't fix the problem with VMAP_STACK.  The buffer could
> still
> > end up spanning two pages and the physical addresses of those pages
> would
> > possibly be discontiguous.  Do you want to just refactor this so that it uses
> a
> > fixed buffer, one that will work with VMAP_STACK?  Or is that coming in a
> future
> > patch?
> 
> Hi Jake, I think the VMAP_STACK issue you mentioned should be another
> different
> issue fixed by Long Li: https://patchwork.ozlabs.org/patch/692447/.
> 
> The VMAP_STACK issue is only an issue when we pass the buffer's physical
> address
> to the hypercall.
> 
> Here the buffer is not passed to any hypercall. We just use
> vmbus_sendpacket()
> to memcpy the buffer into the per-channel ringbuffer.
> 
> Thanks,
> -- Dexuan

Yes, you're right.  This patch looks fine to me.  Sorry for confusing the two issues.

-- Jake