RE: [PATCH] video: hyperv_fb: Fix the cache type when mapping the VRAM

[Haiyang Zhang <haiyangz@microsoft.com>]

> -----Original Message-----
> From: Dexuan Cui <decui@microsoft.com>
> Sent: Tuesday, November 17, 2020 7:03 PM
> To: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
> <haiyangz@microsoft.com>; Stephen Hemminger
> <sthemmin@microsoft.com>; wei.liu@kernel.org;
> b.zolnierkie@samsung.com; linux-hyperv@vger.kernel.org; dri-
> devel@lists.freedesktop.org; linux-fbdev@vger.kernel.org; linux-
> kernel@vger.kernel.org; Michael Kelley <mikelley@microsoft.com>
> Cc: Wei Hu <weh@microsoft.com>; Dexuan Cui <decui@microsoft.com>
> Subject: [PATCH] video: hyperv_fb: Fix the cache type when mapping the
> VRAM
> 
> 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@gmail.com>
> Signed-off-by: Dexuan Cui <decui@microsoft.com>
> ---
> 
> Hi Wei Liu, can you please pick this up into the hyperv/linux.git tree's hyperv-
> fixes branch? I really hope this patch can be in v5.10 since it fixes a
> longstanding issue:
> https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgith
> ub.com%2FLIS%2Flis-
> next%2Fissues%2F655&amp;data=04%7C01%7Chaiyangz%40microsoft.com%
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> 
>  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 5bc86f481a78..c8b0ae676809
> 100644
> --- a/drivers/video/fbdev/hyperv_fb.c
> +++ b/drivers/video/fbdev/hyperv_fb.c
> @@ -1093,7 +1093,12 @@ 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;

Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Thank you!