From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:36099) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aDWuS-0006hk-Rv for qemu-devel@nongnu.org; Mon, 28 Dec 2015 07:28:21 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aDWuP-0003uh-Io for qemu-devel@nongnu.org; Mon, 28 Dec 2015 07:28:20 -0500 Received: from [59.151.112.132] (port=59003 helo=heian.cn.fujitsu.com) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aDWuO-0003uI-Ss for qemu-devel@nongnu.org; Mon, 28 Dec 2015 07:28:17 -0500 References: <1451271263-8275-1-git-send-email-caoj.fnst@cn.fujitsu.com> <20151228115042.GB18063@redhat.com> From: Cao jin Message-ID: <56812B6B.3020105@cn.fujitsu.com> Date: Mon, 28 Dec 2015 20:30:35 +0800 MIME-Version: 1.0 In-Reply-To: <20151228115042.GB18063@redhat.com> Content-Type: text/plain; charset="windows-1252"; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] bugfix: passing reference instead of value List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org, stefano.stabellini@eu.citrix.com On 12/28/2015 07:50 PM, Michael S. Tsirkin wrote: > On Mon, Dec 28, 2015 at 10:54:23AM +0800, Cao jin wrote: >> Fix the bug introduced by 595a4f07. Function host_pci_config_read() should be >> passed by a reference, not a value, for the later pci_default_write_config(). > > What's the effect of the bug? Does it break igd assignment? > How come it worked for people? > If the function is never called, mayber we can get rid > of it completely? > sorry if I didn`t explain it clearly to you. let me try the explanation again: This function is called only when using TYPE_IGD_PASSTHROUGH_I440FX_PCI_DEVICE(when realize it) the effect of the bug: pci_default_write_config(pci_dev, pos, val, len); ^ *its value is always 0* I think it won`t break igd assignment, but just give a wrong register value(forever 0) in PCI config space(wrong register value may results in abnormal working state?). the register should get its value by host_pci_config_read() Because my bad English description, Let me do a analogy, here is the imitation of original code: void swap(unsigned int val) //this is host_pci_config_read() { unsigned int org = 2; memcpy(&val, &org, sizeof(unsigned int)); } int main() { unsigned int val = 0; swap(val); printf("val = %d\n", val); return 0; } author want to get: val = 2. but it will always: val = 0; This is exactly the bug I find. > Stefano? > >> >> Signed-off-by: Cao jin >> --- >> Separated from previous "igd-passthru convert to realize" patch. Since these >> two don`t have dependency, can send it solely. >> >> Not test since it is easy to find out if reading carefully, just compiled. >> >> hw/pci-host/piix.c | 8 +++++--- >> 1 file changed, 5 insertions(+), 3 deletions(-) >> >> diff --git a/hw/pci-host/piix.c b/hw/pci-host/piix.c >> index 715208b..924f0fa 100644 >> --- a/hw/pci-host/piix.c >> +++ b/hw/pci-host/piix.c >> @@ -761,7 +761,7 @@ static const IGDHostInfo igd_host_bridge_infos[] = { >> {0xa8, 4}, /* SNB: base of GTT stolen memory */ >> }; >> >> -static int host_pci_config_read(int pos, int len, uint32_t val) >> +static int host_pci_config_read(int pos, int len, uint32_t *val) >> { >> char path[PATH_MAX]; >> int config_fd; >> @@ -784,12 +784,14 @@ static int host_pci_config_read(int pos, int len, uint32_t val) >> ret = -errno; >> goto out; >> } >> + >> do { >> - rc = read(config_fd, (uint8_t *)&val, len); >> + rc = read(config_fd, (uint8_t *)val, len); >> } while (rc < 0 && (errno == EINTR || errno == EAGAIN)); >> if (rc != len) { >> ret = -errno; >> } >> + >> out: >> close(config_fd); >> return ret; >> @@ -805,7 +807,7 @@ static int igd_pt_i440fx_initfn(struct PCIDevice *pci_dev) >> for (i = 0; i < num; i++) { >> pos = igd_host_bridge_infos[i].offset; >> len = igd_host_bridge_infos[i].len; >> - rc = host_pci_config_read(pos, len, val); >> + rc = host_pci_config_read(pos, len, &val); >> if (rc) { >> return -ENODEV; >> } >> -- >> 2.1.0 >> >> > > > . > -- Yours Sincerely, Cao Jin