linux-kernel - Re: [PATCH] xen/pciback: Don't disable PCI

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Date:   Fri, 15 Feb 2019 11:41:43 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Prarit Bhargava <prarit@...hat.com>, linux-kernel@...r.kernel.org
Cc:     xen-devel@...ts.xenproject.org, jbeulich@...e.com,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH] xen/pciback: Don't disable PCI_COMMAND on PCI device
 reset.

On Wed, Feb 13, 2019 at 06:21:31PM -0500, Prarit Bhargava wrote:
> From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> 

+LKML
> This was submitted in 2015 here
> 
> https://marc.info/?l=linux-kernel&m=142807132515973&w=2
> 
> and has been included in Fedora builds ever since.  No issues have been
> reported with the patch.
> 
> P.
> 
> ----8<----
> 
> There is no need for this at all. Worst it means that if
> the guest tries to write to BARs it could lead (on certain
> platforms) to PCI SERR errors.
> 
> Please note that with af6fc858a35b90e89ea7a7ee58e66628c55c776b
> "xen-pciback: limit guest control of command register"
> a guest is still allowed to enable those control bits (safely), but
> is not allowed to disable them and that therefore a well behaved
> frontend which enables things before using them will still
> function correctly.
> 
> This is done via an write to the configuration register 0x4 which
> triggers on the backend side:
> command_write
>   \- pci_enable_device
>      \- pci_enable_device_flags
>         \- do_pci_enable_device
>            \- pcibios_enable_device
>               \-pci_enable_resourcess
>                 [which enables the PCI_COMMAND_MEMORY|PCI_COMMAND_IO]
> 
> However guests (and drivers) which don't do this could cause
> problems, including the security issues which XSA-120 sought
> to address.
> 
> Reported-by: Jan Beulich <jbeulich@...e.com>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Reviewed-by: Prarit Bhargava <prarit@...hat.com>
> Cc: Juergen Gross <jgross@...e.com>
> ---
>  drivers/xen/xen-pciback/pciback_ops.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/xen/xen-pciback/pciback_ops.c b/drivers/xen/xen-pciback/pciback_ops.c
> index ea4a08b83fa0..787966f44589 100644
> --- a/drivers/xen/xen-pciback/pciback_ops.c
> +++ b/drivers/xen/xen-pciback/pciback_ops.c
> @@ -127,8 +127,6 @@ void xen_pcibk_reset_device(struct pci_dev *dev)
>  		if (pci_is_enabled(dev))
>  			pci_disable_device(dev);
>  
> -		pci_write_config_word(dev, PCI_COMMAND, 0);
> -
>  		dev->is_busmaster = 0;
>  	} else {
>  		pci_read_config_word(dev, PCI_COMMAND, &cmd);
> -- 
> 2.18.1
>