ACK: [bionic:linux-azure-4.15, focal:linux-azure, groovy:linux-azure][PATCH] video: hyperv_fb: Fix the cache type when mapping the VRAM

[William Breathitt Gray]

On Mon, Jan 18, 2021 at 06:26:28PM -0300, Marcelo Henrique Cerri wrote:
> From: Dexuan Cui <decui at microsoft.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1908569
> 
> x86 Hyper-V used to essentially always overwrite the effective cache type
> of guest memory accesses to WB. This was problematic in cases where there
> is a physical device assigned to the VM, since that often requires that
> the VM should have control over cache types. Thus, on newer Hyper-V since
> 2018, Hyper-V always honors the VM's cache type, but unexpectedly Linux VM
> users start to complain that Linux VM's VRAM becomes very slow, and it
> turns out that Linux VM should not map the VRAM uncacheable by ioremap().
> Fix this slowness issue by using ioremap_cache().
> 
> On ARM64, ioremap_cache() is also required as the host also maps the VRAM
> cacheable, otherwise VM Connect can't display properly with ioremap() or
> ioremap_wc().
> 
> With this change, the VRAM on new Hyper-V is as fast as regular RAM, so
> it's no longer necessary to use the hacks we added to mitigate the
> slowness, i.e. we no longer need to allocate physical memory and use
> it to back up the VRAM in Generation-1 VM, and we also no longer need to
> allocate physical memory to back up the framebuffer in a Generation-2 VM
> and copy the framebuffer to the real VRAM. A further big change will
> address these for v5.11.
> 
> Fixes: 68a2d20b79b1 ("drivers/video: add Hyper-V Synthetic Video Frame Buffer Driver")
> Tested-by: Boqun Feng <boqun.feng at gmail.com>
> Signed-off-by: Dexuan Cui <decui at microsoft.com>
> Reviewed-by: Michael Kelley <mikelley at microsoft.com>
> Reviewed-by: Haiyang Zhang <haiyangz at microsoft.com>
> Link: https://lore.kernel.org/r/20201118000305.24797-1-decui@microsoft.com
> Signed-off-by: Wei Liu <wei.liu at kernel.org>
> (cherry picked from commit 5f1251a48c17b54939d7477305e39679a565382c)
> Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri at canonical.com>
> ---
>  drivers/video/fbdev/hyperv_fb.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/video/fbdev/hyperv_fb.c b/drivers/video/fbdev/hyperv_fb.c
> index fe4731f97df7..aad4eea522a9 100644
> --- a/drivers/video/fbdev/hyperv_fb.c
> +++ b/drivers/video/fbdev/hyperv_fb.c
> @@ -705,7 +705,12 @@ static int hvfb_getmem(struct hv_device *hdev, struct fb_info *info)
>  		goto err1;
>  	}
>  
> -	fb_virt = ioremap(par->mem->start, screen_fb_size);
> +	/*
> +	 * Map the VRAM cacheable for performance. This is also required for
> +	 * VM Connect to display properly for ARM64 Linux VM, as the host also
> +	 * maps the VRAM cacheable.
> +	 */
> +	fb_virt = ioremap_cache(par->mem->start, screen_fb_size);
>  	if (!fb_virt)
>  		goto err2;
>  
> -- 
> 2.25.1
> 
> 
> -- 
> kernel-team mailing list
> kernel-team at lists.ubuntu.com
> https://lists.ubuntu.com/mailman/listinfo/kernel-team

Acked-by: William Breathitt Gray <william.gray at canonical.com>
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