linux-kernel - Re: [PATCH 7/7] KVM: arm64: guest debug, HW assisted debug support

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Message-ID: <87oarn1t6h.fsf@linaro.org>
Date:	Mon, 01 Dec 2014 11:54:46 +0000
From:	Alex Bennée <alex.bennee@...aro.org>
To:	Christoffer Dall <christoffer.dall@...aro.org>
Cc:	kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.cs.columbia.edu, marc.zyngier@....com,
	peter.maydell@...aro.org, agraf@...e.de, jan.kiszka@...mens.com,
	dahi@...ux.vnet.ibm.com, r65777@...escale.com, bp@...e.de,
	pbonzini@...hat.com, Gleb Natapov <gleb@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	"open list\:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list\:ABI\/API" <linux-api@...r.kernel.org>
Subject: Re: [PATCH 7/7] KVM: arm64: guest debug, HW assisted debug support


Christoffer Dall <christoffer.dall@...aro.org> writes:

> On Tue, Nov 25, 2014 at 04:10:05PM +0000, Alex Bennée wrote:
<snip>
>> --- a/arch/arm64/kvm/hyp.S
>> +++ b/arch/arm64/kvm/hyp.S
>> @@ -18,6 +18,7 @@
>>  #include <linux/linkage.h>
>>  #include <linux/kvm.h>
>>  
>> +#include <uapi/asm/kvm.h>
>>  #include <asm/assembler.h>
>>  #include <asm/memory.h>
>>  #include <asm/asm-offsets.h>
>> @@ -174,6 +175,7 @@
>>  	ldr	x3, [x0, #GUEST_DEBUG]
>>  	tbz	x3, #KVM_GUESTDBG_ENABLE_SHIFT, 2f	// No guest debug
>>  
>> +	// Both Step and HW BP/WP ops need to modify spsr_el2 and mdscr_el1
>>  	// x0 - preserved as VCPU ptr
>>  	// x1 - spsr
>>  	// x2 - mdscr
>> @@ -191,6 +193,11 @@
>>  	eor	x1, x1, #DBG_SPSR_SS
>>  	eor	x2, x2, #DBG_MDSCR_SS
>>  1:
>> +	// If we are doing HW BP/WP - set MDSCR_EL1.KDE/MDE
>> +	tbz	x3, #KVM_GUESTDBG_USE_HW_BP_SHIFT, 3f
>> +	orr	x2, x2, #DBG_MDSCR_KDE
>> +	orr	x2, x2, #DBG_MDSCR_MDE
>> +3:
>>  	msr	spsr_el2, x1
>>  	msr	mdscr_el1, x2
>>  2:
>> @@ -815,6 +822,33 @@ __restore_debug:
>>  
>>  	ret
>>  
>> +/* Setup debug state for debug of guest */
>> +__setup_debug:
>> +	// x0: vcpu base address
>> +	// x3: ptr to guest registers passed to setup_debug_registers
>> +	// x5..x20/x26: trashed
>> +
>> +	mrs	x26, id_aa64dfr0_el1
>> +	ubfx	x24, x26, #12, #4	// Extract BRPs
>> +	ubfx	x25, x26, #20, #4	// Extract WRPs
>> +	mov	w26, #15
>> +	sub	w24, w26, w24		// How many BPs to skip
>> +	sub	w25, w26, w25		// How many WPs to skip
>> +
>> +	mov     x4, x24
>> +	add	x3, x0, #GUEST_DEBUG_BCR
>> +	setup_debug_registers dbgbcr
>> +	add	x3, x0, #GUEST_DEBUG_BVR
>> +	setup_debug_registers dbgbvr
>> +
>> +	mov     x4, x25
>> +	add	x3, x0, #GUEST_DEBUG_WCR
>> +	setup_debug_registers dbgwcr
>> +	add	x3, x0, #GUEST_DEBUG_WVR
>> +	setup_debug_registers dbgwvr
>> +
>> +	ret
>> +
>>  __save_fpsimd:
>>  	save_fpsimd
>>  	ret
>> @@ -861,6 +895,13 @@ ENTRY(__kvm_vcpu_run)
>>  	bl __restore_sysregs
>>  	bl __restore_fpsimd
>>  
>> +        // Now is the time to set-up the debug registers if we
>> +        // are debugging the guest
>> +	ldr	x3, [x0, #GUEST_DEBUG]
>> +	tbz	x3, #KVM_GUESTDBG_USE_HW_BP_SHIFT, 2f
>> +	bl	__setup_debug
>> +	b	1f
>> +2:
>>  	skip_debug_state x3, 1f
>>  	bl	__restore_debug
>>  1:
>> @@ -881,6 +922,11 @@ __kvm_vcpu_return:
>>  	bl __save_fpsimd
>>  	bl __save_sysregs
>>  
>> +	// If we are debugging the guest don't save debug registers
>> +	// otherwise we'll be trashing are only good copy we have.
>> +	ldr	x3, [x0, #GUEST_DEBUG]
>> +	tbnz	x3, #KVM_GUESTDBG_USE_HW_BP_SHIFT, 1f
>> +
>
> we're introducing an awful lot of conditionals in the assembly code with
> these patches, can you re-consider if there's a cleaner abstraction that
> allows us to deal with some of this stuff in C-code?

See previous mail. It would be good but we need a place to do it before
we enter hyp.S on a KVM_RUN ioctl. I'm open to suggestions.

>
> -Christoffer

-- 
Alex Bennée
--
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