Re: [PATCH v4 04/13] ARM: KVM: Initial VGIC MMIO support code

[Marc Zyngier <marc.zyngier@arm.com>]

On 28/11/12 13:09, Will Deacon wrote:
> On Sat, Nov 10, 2012 at 03:44:44PM +0000, Christoffer Dall wrote:
>> From: Marc Zyngier <marc.zyngier@arm.com>
>>
>> Wire the initial in-kernel MMIO support code for the VGIC, used
>> for the distributor emulation.
> 
> [...]
> 
>> diff --git a/arch/arm/kvm/vgic.c b/arch/arm/kvm/vgic.c
>> new file mode 100644
>> index 0000000..26ada3b
>> --- /dev/null
>> +++ b/arch/arm/kvm/vgic.c
>> @@ -0,0 +1,138 @@
>> +/*
>> + * Copyright (C) 2012 ARM Ltd.
>> + * Author: Marc Zyngier <marc.zyngier@arm.com>
>> + *
>> + * This program is free software; you can redistribute it and/or modify
>> + * it under the terms of the GNU General Public License version 2 as
>> + * published by the Free Software Foundation.
>> + *
>> + * This program is distributed in the hope that it will be useful,
>> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
>> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> + * GNU General Public License for more details.
>> + *
>> + * You should have received a copy of the GNU General Public License
>> + * along with this program; if not, write to the Free Software
>> + * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
>> + */
>> +
>> +#include <linux/kvm.h>
>> +#include <linux/kvm_host.h>
>> +#include <linux/interrupt.h>
>> +#include <linux/io.h>
>> +#include <asm/kvm_emulate.h>
>> +
>> +#define ACCESS_READ_VALUE	(1 << 0)
>> +#define ACCESS_READ_RAZ		(0 << 0)
>> +#define ACCESS_READ_MASK(x)	((x) & (1 << 0))
>> +#define ACCESS_WRITE_IGNORED	(0 << 1)
>> +#define ACCESS_WRITE_SETBIT	(1 << 1)
>> +#define ACCESS_WRITE_CLEARBIT	(2 << 1)
>> +#define ACCESS_WRITE_VALUE	(3 << 1)
>> +#define ACCESS_WRITE_MASK(x)	((x) & (3 << 1))
>> +
>> +/**
>> + * vgic_reg_access - access vgic register
>> + * @mmio:   pointer to the data describing the mmio access
>> + * @reg:    pointer to the virtual backing of the vgic distributor struct
>> + * @offset: least significant 2 bits used for word offset
>> + * @mode:   ACCESS_ mode (see defines above)
>> + *
>> + * Helper to make vgic register access easier using one of the access
>> + * modes defined for vgic register access
>> + * (read,raz,write-ignored,setbit,clearbit,write)
>> + */
>> +static void vgic_reg_access(struct kvm_exit_mmio *mmio, u32 *reg,
>> +			    u32 offset, int mode)
>> +{
>> +	int word_offset = offset & 3;
> 
> You can get rid of this variable.
> 
>> +	int shift = word_offset * 8;
> 
> shift = (offset & 3) << 3;
> 
>> +	u32 mask;
>> +	u32 regval;
>> +
>> +	/*
>> +	 * Any alignment fault should have been delivered to the guest
>> +	 * directly (ARM ARM B3.12.7 "Prioritization of aborts").
>> +	 */
>> +
>> +	mask = (~0U) >> (word_offset * 8);
> 
> then use shift here.

Sure.

>> +	if (reg)
>> +		regval = *reg;
>> +	else {
> 
> Use braces for the if clause.

Indeed.

> 
>> +		BUG_ON(mode != (ACCESS_READ_RAZ | ACCESS_WRITE_IGNORED));
>> +		regval = 0;
>> +	}
>> +
>> +	if (mmio->is_write) {
>> +		u32 data = (*((u32 *)mmio->data) & mask) << shift;
>> +		switch (ACCESS_WRITE_MASK(mode)) {
>> +		case ACCESS_WRITE_IGNORED:
>> +			return;
>> +
>> +		case ACCESS_WRITE_SETBIT:
>> +			regval |= data;
>> +			break;
>> +
>> +		case ACCESS_WRITE_CLEARBIT:
>> +			regval &= ~data;
>> +			break;
>> +
>> +		case ACCESS_WRITE_VALUE:
>> +			regval = (regval & ~(mask << shift)) | data;
>> +			break;
>> +		}
>> +		*reg = regval;
>> +	} else {
>> +		switch (ACCESS_READ_MASK(mode)) {
>> +		case ACCESS_READ_RAZ:
>> +			regval = 0;
>> +			/* fall through */
>> +
>> +		case ACCESS_READ_VALUE:
>> +			*((u32 *)mmio->data) = (regval >> shift) & mask;
>> +		}
>> +	}
>> +}
> 
> It might be a good idea to have some port accessors for mmio->data otherwise
> you'll likely get endianness issues creeping in.

Aarrgh! This depends on the relative endianess of host and guest. 'm
feeling slightly sick...

>> +/* All this should be handled by kvm_bus_io_*()... FIXME!!! */
> 
> I don't follow this comment :) Can you either make it clearer (and less
> alarming!) or just drop it please?

The non-alarming version should read:

/*
 * I would have liked to use the kvm_bus_io_*() API instead, but
 * it cannot cope with banked registers (only the VM pointer is
 * passed around, and we need the vcpu). One of these days, someone
 * please fix it!
 */

> 
>> +struct mmio_range {
>> +	unsigned long base;
>> +	unsigned long len;
>> +	bool (*handle_mmio)(struct kvm_vcpu *vcpu, struct kvm_exit_mmio *mmio,
>> +			    u32 offset);
>> +};
> 
> Why not make offset a phys_addr_t?

Very good point.

>> +static const struct mmio_range vgic_ranges[] = {
>> +	{}
>> +};
>> +
>> +static const
>> +struct mmio_range *find_matching_range(const struct mmio_range *ranges,
>> +				       struct kvm_exit_mmio *mmio,
>> +				       unsigned long base)
>> +{
>> +	const struct mmio_range *r = ranges;
>> +	unsigned long addr = mmio->phys_addr - base;
> 
> Same here, I don't think we want to truncate everything to unsigned long.

Indeed.

>> +	while (r->len) {
>> +		if (addr >= r->base &&
>> +		    (addr + mmio->len) <= (r->base + r->len))
>> +			return r;
>> +		r++;
>> +	}
> 
> Hmm, does this work correctly for adjacent mmio devices where addr sits
> on the boundary (i.e. the first address of the second device)?

I think it is OK. We basically check that both ends of the access are
within a single range (we do not bother with cross range accesses).

	M.
-- 
Jazz is not dead. It just smells funny...