Re: [Qemu-devel] [PATCH v2 5/6] vga: Use linear mapping + dirty logging in chain 4 memory access mode

[Avi Kivity <avi@redhat.com>]

On 08/17/2011 03:41 PM, Jan Kiszka wrote:
> From: Jan Kiszka<jan.kiszka@siemens.com>
>
> Most VGA memory access modes require MMIO handling as they demand weird
> logic to get a byte from or into the video RAM. However, there is one
> exception: chain 4 mode with all memory planes enabled for writing. This
> mode actually allows lineary mapping, which can then be combined with
> dirty logging to accelerate KVM.
>
> This patch accelerates specifically VBE accesses like they are used by
> grub in graphical mode. Not only the standard VGA adapter benefits from
> this, also vmware and spice in VGA mode.

Excellent, this slowness is very annoying.

>
> +static void vga_update_memory_access(VGACommonState *s)
> +{
> +    MemoryRegion *region, *old_region = s->chain4_alias;
> +    target_phys_addr_t base, offset, size;
> +
> +    s->chain4_alias = NULL;
> +
> +    if ((s->sr[0x02]&  0xf) == 0xf&&  s->sr[0x04]&  0x08) {
> +        offset = 0;
> +        switch ((s->gr[6]>>  2)&  3) {
> +        case 0:
> +            base = 0xa0000;
> +            size = 0x20000;
> +            break;
> +        case 1:
> +            base = 0xa0000;
> +            size = 0x10000;
> +            offset = s->bank_offset;
> +            break;
> +        case 2:
> +            base = 0xb0000;
> +            size = 0x8000;
> +            break;
> +        case 3:
> +            base = 0xb8000;
> +            size = 0x8000;
> +            break;
> +        }
> +        region = qemu_malloc(sizeof(*region));
> +        memory_region_init_alias(region, "vga.chain4",&s->vram, offset, size);
> +        memory_region_add_subregion_overlap(get_system_memory(), base,
> +                                            region, 2);

get_system_memory() has been excised from vga in batch 4 of my patches, 
please adjust.

> +        s->chain4_alias = region;
> +    }
> +    if (old_region) {
> +        memory_region_del_subregion(get_system_memory(), old_region);
> +        memory_region_destroy(old_region);
> +        qemu_free(old_region);
> +    }
> +}
> +
>

The mmio code has

             s->plane_updated |= mask; /* only used to detect font change */

aren't we losing it?  we could easily recover it via dirty logging.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.