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Re: [Xen-users] Theorycraft about Radeons VGA Passthrough issues


Just wondering if you ever got get any replies to this email?


-- Pasi

On Mon, Sep 15, 2014 at 04:58:03AM -0300, Zir Blazer wrote:
>    I think I have some interesing info regarding the infamous Radeon's
>    "performance glitch" issue.
>    I have a Radeon 5770, a Sapphire Flex Edition to be specific, should be
>    this one:
> [1]http://www.sapphiretech.com/presentation/product/product_index.aspx?psn=0001&pid=355&lid=1
>    It had by default a highest Power State that runs the GPU @ 850 MHz, VRAM
>    @ 1200 MHz, and the GPU Voltage is 1.125V, if I recall correctly. Because
>    I usually underclock/undervolt absolutely everything in the name of power
>    efficiency, I had modified the Video Card BIOS with Radeon BIOS Editor
>    with custom Power States. You can actually flash it from within a VM - I
>    could flash mine with the ATI WinFlash tools. However, you need a computer
>    reboot for changes to take effect, restarting the VM isn't enough. My
>    current PowerPlay settings are these, along with a modified Fan curve:
>    [2]http://i.imgur.com/YKuS1pa.png
>    The VM where I use this Radeon uses WXP SP3 with the GPLPV Drivers
> I use Arch Linux as Dom0 with Xen 4.3.1 builded with the ATI
>    VGA Passthrough patch that was included in the Arch Linux User Repository
>    xen package (This patch is also currently provided for Xen 4.4.1, it
>    builds properly with it but its claimed to not have been tested). Syslinux
>    is the Boot Loader, and I have specified in its config file to hide the
>    Radeon PCI address, so Dom0 shouldn't see or initialize it.
>    On a "good" VM start, I can run GPU-Z and it will see how my Video Card
>    switchs to different PowerStates (The ones I configured previously)
>    depending on load. That's good.
>    [3]http://i.imgur.com/VeHZkhy.gif
>    [4]http://i.imgur.com/kCLSkh3.gif
>    Sometimes, like when I shut down the VM then open it again, the Radeon
>    gets stuck on a Power State which is NOT a value from the PowerPlay table
>    which I modified. In my Video Card, it is GPU @ 850 MHz with VRAM @ 1200
>    MHz, which was the highest default Power State on the original BIOS, but
>    it is not present in mine anymore. Also, GPU-Z and other tools fails to
>    report GPU Voltage, which I suppose should be 1.125V. Temperatures on load
>    are also accordingly much higher than what archivable with my highest
>    Power State, so does Fan noise, so I suppose than the Video Card is really
>    running at those values, through I didn't benchmarked them (Should be
>    faster for obvious reasons).
>    [5]http://i.imgur.com/F5S0giC.gif
>    I suppose that when the Video Card fails to be fully initialized properly,
>    instead it falls back on a "backup" Power State, which in my Video Card
>    coincided with the highest one, and that Power State is not part of the
>    regular PowerPlay table. My theory is that when you have the "performance
>    glitchs", it is because other BIOSes may instead have a backup Power State
>    which should be close to what you expect of a power saving mode, while in
>    my case it is totally the opposite.
>    While some people claims that you need to do a full reboot of the computer
>    to do a reboot of the VM that uses the Radeon, I didn't had such types of
>    issues, is as simple as restarting the VM one or two more times, and as
>    soon as GPU-Z shows 150/300 or 700/800 I know it is good to go. I *DID*
>    needed full computer reboot for VBIOS flashs to take effect, and while
>    experimenting Frequency/Voltage settings, if the Voltage was too low to be
>    fully stable at that Frequency, I couldn't get the GPU to work again
>    without a computer reboot, with the VM always BSODing on boot.
>    I recall having seen BIOSes in TechPower VGA BIOS Collection whose
>    PowerPlay tables had some weird values, like for example, Frequencies
>    appropiate for Idle with highest Power State GPU Voltage, and more
>    interesing, viceversa, which should be a no-go. Due to the fact that I
>    don't know what the "backup" settings are for when the Video Card doesn't
>    fully initialize properly, nor where they come from and if they are
>    rational values, I suspect that a bad combination of those hidden values
>    could be heavily related to this and is why these people have this issue.
>    Sometimes I have experienced crashes on the WXP VM that forces me to kill
>    it from Dom0 with xl, and that leaves the screen on the Monitor attached
>    to the Radeon with a frozen screen. The next VM start it displays some
>    weird behaviator, because the Monitor gets refreshed with a white screen,
>    after some time it goes black, then some time after a BSOD on Dom0 VM's
>    screen follows. However, restarting the VM again, it works properly. So
>    for pretty much any non-GPU stability related issue, closing and opening
>    the VM a few times gets the Video Card initilized properly sooner or
>    later. So in the last 4-5 months of usage after finishing my PowerPlay
>    table which proved to be stable, there has been no time where I had to
>    actually reboot the computer to restart the VM in any event (Normal shut
>    down then creating it again with xl, or killing the VM due crash or
>    whatever).
>    I'm interesng in people that also had issues with VGA Passthrough and
>    Radeons to post GPU-Z screens (Or write the values) when they suffer from
>    performance degradation after VM restarts. I'm inclined to believe than it
>    is entirely related to the vBIOS backup Power State that it uses when it
>    fails to initialize properly.
> References
>    Visible links
>    1. 
> http://www.sapphiretech.com/presentation/product/product_index.aspx?psn=0001&pid=355&lid=1
>    2. http://i.imgur.com/YKuS1pa.png
>    3. http://i.imgur.com/VeHZkhy.gif
>    4. http://i.imgur.com/kCLSkh3.gif
>    5. http://i.imgur.com/F5S0giC.gif

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