From: Dexuan Cui <decui@xxxxxxxxxxxxx> Sent: Tuesday, November 17, 2020 4:03 PM
>
> 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@xxxxxxxxx>
> Signed-off-by: Dexuan Cui <decui@xxxxxxxxxxxxx>
> ---
>
> 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://github.com/LIS/lis-next/issues/655
>
> 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;
>
> --
> 2.19.1
Reviewed-by: Michael Kelley <mikelley@xxxxxxxxxxxxx>
