Re: [PATCH v2 22/39] xen/riscv: add guest memory read helper

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 9/9/26 2:04 PM, Baptiste Le Duc wrote:
> > Introduce riscv_read_guest() to allow Xen to safely read guest memory
> > using HLV/HLVX instructions while reliably capturing trap context.
> > This is required for instruction fetch emulation and MMIO decoding, where
> > Xen must inspect guest memory that may not be directly accessible and may
> > fault.
> >
> > The implementation is based on kvm_riscv_vcpu_unpriv_read() from Linux,
> > with one deviation: the hlv/hlvx instructions translate the guest address
> > through the live vsatp/hgatp CSRs, i.e. through the address space of the
> > currently running vCPU, so the function can only be called safely for
> > current. Instead of taking a struct vcpu argument, it always operates on
> > current directly.
> >
> > Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
> >
> > diff --git a/xen/arch/riscv/guestcopy.c b/xen/arch/riscv/guestcopy.c
> > index 8a89212e0b..b2327822ac 100644
> > --- a/xen/arch/riscv/guestcopy.c
> > +++ b/xen/arch/riscv/guestcopy.c
> > @@ -6,6 +6,7 @@
> >   #include <xen/string.h>
> >   
> >   #include <asm/guest_access.h>
> > +#include <asm/traps.h>
> >   
> >   #define COPY_from_guest     0U
> >   #define COPY_to_guest       BIT(0, U)
> > @@ -114,3 +115,89 @@ unsigned long copy_to_guest_phys(struct domain *d, paddr_t 
> > gpa, void *buf,
> >       return copy_guest(buf, gpa, len, GPA_INFO(d),
> >                         COPY_to_guest | COPY_gpa);
> >   }
> > +
> > +/*
> > + * Read machine word from guest memory
> > + *
> > + * @guest_addr: Guest address to read
> > + * @read_insn: Flag representing whether we are reading instruction
> > + * @trap: Output pointer to trap details if something went wrong during read
> > + *
> > + * The hlv/hlvx instructions translate guest_addr through the live
> > + * vsatp/hgatp CSRs, so the read is only meaningful for the address
> > + * space of the currently running vCPU.
> > + *
> > + * At most two halfwords are fetched when @read_insn is true, i.e. encodings
> > + * wider than 32 bits are not supported. Such an encoding cannot be completed
> > + * by calling this function again at @guest_addr + 4: the length check is
> > + * applied to the first halfword read, which would then be a continuation of
> > + * the instruction rather than its opcode. It is up to the caller to reject
> > + * anything that is neither a 16- nor a 32-bit encoding.
> > + */
> > +unsigned long riscv_read_guest(unsigned long guest_addr, bool read_insn,
> > +                               struct trap_info *trap)
> Nit: every other function in this file / declared in this header
> (raw_copy_from_guest, copy_to_guest_phys, ...) has no riscv_ prefix. Why
> does this one get it?
Agree not to much sense. I will drop it.

> > +{
> > +    /*
> > +     * Poison the result: if the very first access faults, the fixup skips
> > +     * over the loads without writing it. Callers must check trap->scause.
> > +     */
> > +    unsigned long val = ~0UL, tmp;
> The actual "did a trap happen" contract callers rely on is
> trap->scause == 0. If the very first access faults, fixup_exception() will
> write scause accordingly to a non-zero value, but nothing in this function
> clears trap->scause on the success path.
It is because caller is expected to zero-fill trap as it is happening 
now. I will mention that explicitly. I will do the following:
- * @trap: Output pointer to trap details if something went wrong during 
read
+ * @trap: Output pointer to trap details if something went wrong during 
read.
+ *        It must be zero-initialised by the caller: it is written only 
when
+ *        an access faults, so trap->scause == 0 on return is what 
tells the
+ *        caller that the read succeeded.


> > +
> > +    /*
> > +     * hlv/hlvx use hstatus.SPVP for the privilege of the access, and the
> > +     * live vsatp/hgatp for the translation. Xen never installs a value of
> > +     * its own in hstatus (it is only saved on trap entry and restored
> > +     * before sret) and it doesn't reschedule before returning to the
> > +     * guest, so all three still belong to the vCPU which trapped.
> > +     *
> > +     * Check the saved copy rather than the live CSR: a nested trap taken
> > +     * from HS-mode clears hstatus.SPV in the CSR (but leaves SPVP alone).
> > +     */
> > +    ASSERT(vcpu_guest_cpu_user_regs(current)->hstatus & HSTATUS_SPV);
> Just to understand, what is the aim of this check? Is it to be sure this
> function has been called during a guest fault and not a nested HS fault?
Yes.

~ Oleksii